BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to anatomical support cushions adapted for use in improving pelvic and spinal alignment issues during use of a wheelchair apparatus. Brief Statement of the Prior Art Problems to be Addressed by the Present Invention Non-ambulatory patients suffering with minor illness or simple leg bone injuries, or the like, do not normally require the use of special purpose wheelchair cushions. However, in those long term care (LTC) cases where the patient suffers from serious postural injury or physical conditions involving pelvic or spinal related abnormalities expressing pelvic obliquity issues, anterior pelvic tilt issues, posterior and/or pelvic tilt issues, or pelvic rotation issues, the use of specially configured cushioning or other body positioning, orienting or supporting means may be required. The present invention is intended to address this need and to provide an adjustable wheelchair cushion that can be specified, prescribed and/or used by LTC therapists to improve the efficacy of wheelchair cushioning used in this environment. The following is included to provide background information relating to some of the postural abnormalities toward which the present invention is directed, and how an LTC therapist has sought to provide, or has in fact provided, patient relief during use of a standard wheelchair and cushioning means. TERMS used herein

LTC: Long Term Care

Lordosis: an abnormal forward curvature of the spine in the lumbar region, resulting in a swaybacked posture.

Kyphosis: an abnormal, convex curvature of the spine, with a resultant bulge at the upper back. Scoliosis: an abnormal lateral curvature of the spine.

Oblique:

1) neither perpendicular nor parallel to a given line or surface; slanting; sloping.

2) (of a solid) not having the axis perpendicular to the plane of the base.

3) diverging from a given straight line or course.

Obliquity: the state of being oblique.

Cervical: of or relating to the cervix or neck.

Lumbar: of or relating to the loin or loins.

Loin: the part or parts of the human body or of quadruped animal

on either side of the spinal column, between the false ribs and hipbone.

Thorax: the part of the trunk in humans and higher vertebrates between

the neck and the abdomen, containing the cavity, enclosed by the ribs,

sternum, and certain vertebrae, in which the heart, lungs, etc., are situated; chest.

Spine: the spinal or vertebral column; backbone.

Sacrum: bone resulting from the fusion of two or more vertebrae between the

lumbar and the coccygeal regions, in humans being composed usually of

five fused vertebrae and forming the posterior wall of the pelvis.

ELR: Elevating Leg rest

STFH: Seat-to-Floor Height

MWC: Manual Wheelchair

IT: Ischial Tuberosity

PSIS: Posterior Superior Iliac Spine

ASIS: Anterior Superior Iliac Spine

LE: Lower Extremity UE: Upper Extremity

ROM: Range of Motion

ADLs: Activities of Daily Living

PPT: Posterior Pelvic Tilt OPTIMAL POSTURE– generally depicted in Fig.2 of the Drawings

Pelvis in midline.

ASIS & PSIS at equal height: no pelvic tilt.

L ASIS & R ASIS at equal height: no obliquity

L ASIS & R ASIS at equal depth: no rotation

Spine balanced and upright; no rotation, no lateral curvature.

Normal lordosis in cervical and lumbar spine and normal kyphosis in thoracic spine creating the desired“S” shape. The head is functionally upright with only mild forward/lateral flexion or rotation.

When fitting an LTC user for a wheelchair, an LTC therapist will usually try to maintain proper patient postural alignment with a STANDARD cushion (and back support). PELVIC OBLIQUITY - generally depicted in Fig. 3 of the Drawings

In patients with this abnormality, the pelvis sits with the L or R ASIS higher than the other, causing the raising up of one hip. When one side of the pelvis is raised higher than the other, the thoracic spine curves away from the higher side creating a scoliosis over time.

The neck will go into lateral flexion as if the person is dropping an ear to his or her shoulder. The lateral flexion will usually be towards the side where the hip is higher.

Pressure redistribution is the therapist’s support goal whether the deformity is FIXED or FLEXIBLE.

If FLEXIBLE: the pelvis should be leveled by building up the lower side.

If FIXED: the deformity should be accommodated by protectjng the bony prominences from pressure by "filling in" the higher side and immersing the lower side IT. ANTERIOR PELVIC TILT - generally depicted in Fig.4 of the Drawings With this abnormality, the pelvis sits with ASIS higher than the PSIS resulting in the posterior pelvic tilt. which produces the sacral sitting posture. With excessive lordosis of the lumbar and cervical spine, the patient hyperextends his or her back over the sling back of the chair, placing him or her at risk to tip the chair backwards.

Excessive lordosis of cervical spine causes hyperextension of the neck and upward eye gaze. With this abnormality, the goal is normally to utilize a cushion and back support that maximizes contact with the seat surface for optimal pelvic and spinal stability and pressure redistribution. Stability is the goal so a back support is normally used that is tall enough for the patient. Measurements are made from seat surface to the top of shoulder. A moldable back support to conform to the curvature of the spine is normally used. POSTERIOR PELVIC TILT - generally depicted in Fig.5 of the Drawings

With this abnormality, the pelvis sits with ASIS higher than the PSIS resulting in the posterior pelvic tilt which produces the sacral sitting posture. Excessive thoracic kyphosis, producing “C” shape spine“flattening out” of the lordosis of the cervical spine and“flattening out” of the lordosis of the lumbar spine.

Decreased lordosis in cervical spine causes forward neck flexion & downward eye gaze to floor/lap. A cushion with medial and lateral contour is often used to promote LE alignment and pelvic stability. Appropriate cushion depth should be used to prevent the patient from sliding forward as he seeks reduced pressure behind the knees.

A rigid insert is often used to prevent hammocking of the seat and cushion and keep the pelvis from collapsing into a posterior pelvic tilt.

If FLEXIBLE: A cushion with tapered adductors can be used to load the trochanters, stabilizing the pelvis in the patient's most neutral alignment.

If FLEXIBLE: A cushion with an anti-thrust component can be used to reduce forward sliding of the pelvis into posterior pelvic tilt.

If FIXED: An immersion style cushion that contours to the shape of the patient used to promote maximum pressure redistribution, minimizing peak pressures.

If FIXED: Opening seat-to-back angle in conjunction with a fixed tilt in the wheelchair is often used to match the patient's ROM limitations and minimize forward sliding. PELVIC ROTATION - generally depicted in Fig. 6 of the Drawings

Pelvis sits with L or R ASIS more forward than the other producing rotation in the hips.

The thoracic spine follows and rotates in the same direction as the pelvis. Therefore, if the right side of the pelvis is rotated more forward, the right side of the spine is rotated more forward as well. The neck will go into lateral flexion as if the person is dropping the ear to his or her shoulder.

In addressing this abnormality, the objective is to stabilize the pelvis in the patient’s most neutral position—adjusting for flexible or fixed postures—to prevent further pelvic rotation. One option is to apply an anti-thrust force to reduce forward migration of the pelvis.

If FLEXIBLE: use tapered adductors and a medial abductor to create leg troughs for midline LE alignment and to maintain a pelvic neutral position.

If FIXED: use a cushion with less aggressive contouring. Protect the bony prominences with an immersion-style cushion. Objects of this Invention An object of the present invention is to provide an adjustable wheelchair cushion apparatus that can be adapted to accommodate the particular anatomical support needs of a user having postural abnormalities. Another object of the present invention is to provide an improved anatomical support cushion apparatus including at least one thermoplastic elastomer honeycomb cushion panel and a plurality of selectable inserts that serve to conform, support and/or stabilize a wide variety of wheelchair user sitting positions, user anatomies and disabilities. Another object of the present invention is to provide an improved adjustable anatomical support cushion apparatus for providing pelvic and trunk stability for a wheelchair user having postural abnormalities. Still another object of the present invention is to provide an adjustable anatomical support cushion apparatus for maximizing functional activities of daily living for a wheelchair user having postural abnormalities. Yet another object of the present invention is to provide an improved anatomical support cushion apparatus for protecting skin and preventing wounds, and/or promoting the healing of existing wounds suffered by a wheelchair user having postural abnormalities. A further object of the present invention is to provide an adjustable support cushion apparatus for maximizing comfort for a wheelchair user having postural abnormalities. Another object of the present invention is to provide an adjustable anatomical support cushion apparatus for minimizing unwanted skeletal movement by a wheelchair user having postural abnormalities. Yet another object of the present invention is to provide an adjustable support cushion for preventing progression of postural abnormalities. A still further object of the present invention is to provide an adjustable anatomical support apparatus that is breathable to permit cooling of the user. Another object of the present invention is to provide an adjustable anatomical support apparatus that can be tailored to create certain desired cushioning and stabilizing characteristics without having to introduce elements such as foam, fluids or other means which add cost or reduce durability. Still another object of the present invention is to provide an adjustable anatomical support apparatus constructed of materials that are fast drying, and can be easily disinfected and sterilized by chemical wash, microwave treatment, detergent wash, or other means. SUMMARY OF THE INVENTION Briefly stated, the present invention is directed to an adjustable wheelchair seat cushion apparatus including a resilient thermoplastic honeycomb seat cushion member and a pivotally attached base member forming a planar base upon which the upper cushion member will rest when folded about the pivot to engage and lay thereupon. The base member may also be made of one or more layers of a more rigid thermoplastic honeycomb material. Positioned upon the base member and disposed between it and the overlying cushion member are user and/or LTC therapist installable, prescriptively sized and shaped pelvic obliquity elements, pommel elements and/or wedge elements operative to deform the cushion member when engaged therewith. Once the obliquity, pommel and/or wedge elements are positioned and affixed to the base member, the cushion member is rotated into engagement therewith and deformed thereby, and the assembly is inserted into its fabric cover, the zipper is retracted and the cushion assembly is ready for use. In some cases, an ischial pad may also be inserted into a pocket provided in the back-center portion of the cover to protect the patient’s ischial bone. An important advantage of the present invention is that the thermoplastic elastomeric honeycomb material used in the construction of the apparatus is an anisotropic material having improved pressure relief, stability, compression set resistance, durability and low maintenance characteristics. Another advantage of the present invention is that in fitting the cushion apparatus for a particular user, an LTC therapist may select from a wide variety of preformed shaping elements and cushion positions therefor to customize and individually tailor the cushion apparatus for an individual user. Yet another advantage of the present invention is that a single size and shape of cushion can be matched with a variety of individually selectable, preformed shaping elements to support the specific requirements of a particular wheelchair user. Still another advantage of the present invention is that the anatomical support apparatus may be constructed from a perforated core thermoplastic elastomer honeycomb panel that is breathable to allow perspiration removal and cooling of the apparatus user. Another advantage of the present invention is that different thermoplastic elastomeric honeycomb core designs and/or multiple panels of different thermoplastic honeycomb core designs may be utilized to maximize design flexibility of the improved anatomical support apparatus. Still another advantage of the present invention is that the thermoplastic elastomeric honeycomb core is fabricated from recyclable materials that are fast drying, and easily disinfected and sterilized. These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the preferred embodiments which are contained in and illustrated by the various drawing figures. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG.1 is a perspective view generally illustrating a standard wheelchair having disposed thereon an improved anatomical support cushion apparatus constructed in accordance with a presently preferred embodiment of the present invention; FIG.2 is a stylized side elevational view illustrating an optimally postured patient sitting upright without cushion in a standard wheelchair of the type shown in FIG.1; FIG.3 is a stylized partial cross-sectional view taken along the line 3--3 of the wheelchair illustrated in FIG.1, and a patient expressing pelvic obliquity abnormality to illustrate one example of a condition that can be addressed using a cushion apparatus in accordance with the present invention; FIG.4 is a stylized side elevational view illustrating a patient sitting in a standard wheelchair of the type shown in FIG.1 and expressing an anterior pelvic tilt abnormality to illustrate another example of a condition that can be addressed using a cushion apparatus in accordance with the present invention; FIG.5 is a stylized side elevational view illustrating a patient sitting in a standard wheelchair of the type shown in FIG.1 and expressing a posterior pelvic tilt abnormality to illustrate another example of a condition that can be addressed using a cushion apparatus in accordance with the present invention; FIG.6 is a partially broken plan view illustrating a patient sitting in a standard wheelchair of the type shown in FIG.1 and expressing a pelvic rotation abnormality to illustrate another example of a condition that can be addressed using a cushion apparatus in accordance with the present invention; FIG.7 is a stylized three-quarter view generally illustrating a cushion assembly, in its open configuration, the user supporting cushion member, the pivotally attached base member, and an assortment of obliquity elements, pommel elements, and wedge elements disposed on the base member in accordance with an embodiment of the present invention; FIGs.8-10 illustrate in 3-dimential perspective, generalized examples of the obliquity elements, pommel elements, and wedge elements depicted in FIG.7; FIG.11 is a perspective view generally illustrating an ischial pad of a configuration that might be installed in an appropriate location in or on either the cushion assembly or its cover; FIG.12 is a stylized three-quarter perspective view generally illustrating, in its open

configuration, a zippered cover envelope for receiving and containing the cushion assembly illustrated in FIG.7; FIG.13 is a stylized three-quarter perspective view, generally illustrating, in its open configuration, as viewed opposite from the upper back side as depicted in FIG.7, of the user supporting cushion member and the pivotally attached base member, in accordance with an embodiment of the present invention; FIG.14 is an enlarged side view of the cushion assembly depicted in its open configuration; FIG.15 is an enlarged side view of the cushion assembly depicted in its closed configuration; FIG.16 is an enlarged frontal view showing the cushion assembly depicted in its closed configuration; and FIG.17 is partially broken side view showing the closed cushion assembly enveloped in its zippered cushion assembly cover. DETAILED DESCRIPTION OF THE EMBODIMENTS Referring now to FIG.1 of the Drawings which, as pointed out above, depicts a

conventional wheelchair 10 having disposed thereon a removable improved anatomical support cushion 12. As will be described in greater detail below, the cushion 12 is constructed of at least one flexible thermoplastic elastomer honeycomb core panel built in accordance with the present invention. It should also be noted that, although the cushion assembly 12 is particularly well suited for wheelchair applications, the cushion assembly, or analogs thereof, may also be used in a variety of other anatomical support applications (e.g., mattresses, automobile and airline seats, arm rests, etc.). Fig.2 is provided to illustrate optimal user posture when seated in a standard wheelchair, Figs.3- 6 are provided to respectively illustrate postural conditions identified as pelvic obliquity, anterior pelvic tilt, posterior pelvic tilt and pelvic rotation; all are conditions that can be aided through use of the present invention. Fig.7 generally depicts the principal components of an adjustable wheelchair seat cushion apparatus 12 including a multi-layer resilient thermoplastic honeycomb seat cushion member 14 and a pivotally attached base member16 forming a planar base upon which the upper cushion member 14 will rest when folded about the pivot 18 to engage and lay thereupon. The base member 16 may also be made of one or more layers of a more rigid thermoplastic honeycomb material. Positioned upon the base member 18 and disposed between it and the overlying cushion member 14 are user and/or LTC therapist installable, prescriptively sized and shaped pelvic obliquity elements 20, pommel elements 22 and/or wedge elements 24 operative to deform the cushion member 14 when engaged therewith. Once the obliquity, pommel and/or wedge elements are positioned and affixed to the base member 16, the cushion member 14 is rotated into engagement therewith and deformed thereby, and the assembly is inserted into its fabric cover 30, (Fig.12), the cover zipper 32 (Fig.11) is retracted and the cushion assembly is ready for use. In some cases, an ischial pad 32 may also be inserted into a pocket 34 provided in the back-center portion of the cover 30 to protect the patient’s ischial bone. FIG.13 illustrates a multi-layer honeycomb panel structure constructed from thermoplastic elastomer materials. The panels includes a honeycomb-like core preferably made of bonded together and expanded strips or ribbons of plastic material to which facing sheets of perhaps heavier gauge material are thermo-compression bonded. The illustrated cover member 14 is an anisotropic three-dimensional structure having predetermined degrees of flex along the X, Y and Z axes. Each cell is formed, in part, by four generally S-shaped wall segments each of which is bonded to and shared by an adjacent cell. In addition, each cell shares a double thickness wall segment with two adjacent cells. Panel 14 has high tear and tensile strength and is highly resilient, with optimal compression load and shock absorption or distortion characteristics, yet is extremely light weight. Selected combinations of elastomer material, honeycomb cell configurations, core thicknesses and facing material variables will determine the panel's characteristics of softness or hardness, resilient recovery rate and rigidity or flex as required for a particular application. The facing materials can be selected from a wide variety of films, including thermoplastic urethanes, foams, EVAs, rubber, neoprene, elastomer impregnated fibers and various fabrics, etc. The manufacturing and fabrication of the panel 14 is described in greater detail in our U.S. Pat. No. 5,039,567 and is expressly incorporated herein by reference. More specifically, the present invention relates to an adjustable seat cushion assembly including an upper component preferably made of multiple layers of resilient thermoplastic honeycomb material, shaped and bonded together to form a user supporting cushion member, and a generally planer lower component preferably made of at least one relatively thin sheet of more rigid honeycomb material, and having an edge thereof hingedly connected to the cushion member, preferably along it's rearmost extremity. The lower component is adapted to form a relatively rigid base member upon which the overlying cushion member will be supported when folded about the hinged connection to lie thereover. Removably disposed between the base member and the cushion member, and preferably attached to the upper surface of the base member at predefined locations, are one or more cushion deforming elements selected to define the user support characteristics of the cushion assembly. With the upper cushion member rotated upwardly relative to the base member, the upper surface of the base member forms a supporting surface for an assortment of prescriptively sized and shaped pelvic obliquity elements, pommel elements and/or wedge elements. These elements are selected, positioned and secured by the user and/or assisting therapist or clinician on the upwardly facing base member surface so as to deform the overlying resilient cushion member to support, raise, align, orient or otherwise adjust the user’s skeletal components supported by the cushion assembly to provide pelvic and trunk stability, maximize the user’s comfort and function during ADLs, protect the user’s skin and prevent wounds, or assist in the healing of existing wounds, minimize unwanted movement of the user on the cushion surface, correct or accommodate postural abnormalities and/or prevent or mediate progression of postural abnormalities. As will be further described below, these elements are adapted to deform the cushion member as it is lowered (folded down) into supporting engagement therewith. The preformed obliquity elements, pommels and wedges are carefully selected and positioned to provide a desired customizing deformation of the overlying cushion member so that the cushion provides the supporting needs of the user when the assembly is folded into its closed, or folded,

configuration. Once folded, the cushion assembly is inserted into an enveloping fabric cover

that is configured to conform to the shape of the cushion assembly and is provided with perimeter zippering means, or the like, to securely house the cushion assembly and maintain it in its closed and functional configuration. The cover may also be provided with internal and/or external pockets for receiving additional ischial pads, pommels or wedges, etc. In Figs.12-17 a multi-layered cushion member embodiment, the details of which are more fully disclosed in Applicant’s (expressly incorporated hereinto by reference) US Patent No.

5,617,595, is shown modified to include a lower flap or base member as described above to more clearly illustrate attachment of the flap edge by thermocompression bonding along the rear side of the upper cushion. member. Alternatively, the flap edge may be otherwise attached to the upper cushion member. In Figs.14 and 15, side views of the above described cushion assembly are shown in more detail. In Fig.16 a frontal view is depicted, and in Fig.17, a partially broken side view is shown further illustrating the cushion assembly disposed within its fabric cover. Note that a closing zipper is provided around three sides of the lower edge of the cover, and a second medially disposed expansion zipper and excess material facility is suggested to accommodate an assembly of larger cushion deforming elements should that be necessary. Although embodiments of the present invention have been disclosed above, it will be appreciated that numerous alterations and modifications thereof will no doubt become apparent to those skilled in the art after having read the above disclosures. For example, the anatomical support cushions may be configured in any appropriate shape, with multiple panels and with various combinations of perforated and non-perforated core panels, and with core walls and/or face sheet perforations the number and/or hole size of which are tailored to achieve desired damping characteristics. It is therefore intended that the following claims may be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.