FIELD OF THE INVENTION The present invention is related to a method of continuous cooking and washing of cellulose-bearing raw material, especially wood chips, by using an improved liquor circulation pattern. More particularly, the present invention relates to a digester provided with the screens and piping necessary for carrying out this method. BACKGROUND OF THE INVENTION A continuous digester for pulp manufacture is generally a vertical, essentially cylindrical vessel, receiving a feed of cellulose-bearing raw material (for example, wood chips) in the top of the digester and discharging cooked chips, i.e. pulp, from the bottom of the digester. The vessel is usually a pressure vessel, although this disclosure is not limited to such vessels. At one or several points along the digester, going downwards from the top, there may be an increase in digester diameter. Adjacent such a diameter increase, and generally below it, there is typically an annular screen structure in the vessel inner wall. Through this screen structure, liquor is removed from the downward moving chip column by means of a pressure difference. This liquor can be returned into the process by pumping it back into the digester in the middle of the descending chip column through a central pipe, either above the screen (whereby co-current circulation is generated) or below the screen (providing counter-current circulation) . The circulating liquor is heated, and appropriate chemicals and liquids may be added to it to achieve the desired chemical and thermal conditions for the chip column moving downwards past the screen structure. The liquor flowing out through the screen structure can also be finally withdrawn from the digester to be reprocessed at another location. As such extracted liquor is no longer returned to the digester, it is replaced with e.g. washing filtrate from a downstream washing plant. The filtrate is generally pumped into the bottom of the pressurized digester. This washing liquor is forced to flow counter-currently upwards through the packed column to exit through the above-mentioned extraction screens, and is further conducted to reprocessing. In a prior art cooking method, part of this washing liquor is added to the circulation flows together with cooking chemicals, and a corresponding amount of liquor is extracted from the circulation screens for reprocessing. The general aim is to replace the liquor affecting the chips passing the circulation screens. In this method, the amount of washing liquor to be added to the digester bottom is correspondingly reduced. This assists the downward movement of the chip column, because the counterforce created by the washing filtrate flowing upward through the chip bed is reduced. A problem associated with the above-mentioned designs is the radial flow of liquor through the chip bed from the central pipe towards the screen structure at the digester wall. The chip column causes a dynamic pressure loss for radial flow, this loss being dependant on the flow rate. This results in a force vector in the radial direction, which force pushes the chip bed against the screen structure. Because the chip bed is flexibly subject to any force, this dynamic radial force causes thickening, i.e. packing of the chip column against the screen structure at the vessel inner wall, which in turn results in an increased dynamic pressure loss, etc. If the screen structure at the digester wall is overloaded by the liquid flow through the chip column, the screen structure is blocked very quickly, and there will be disturbances in the process; the movement of the chip column may even stop, resulting in production losses. When a screen as described above is used for extraction only, without a circulation flow to displace the extraction liquor, a zone of zero liquid velocity appears in the middle of the digester. Therefore, in the counter-current washing zone of the digester, the washing efficiency remains lower in the middle of the digester than close to the digester wall. There have been attempts to solve this problem using a so-called quench circulation. The washing liquor, flowing from the digester bottom upwards to the extraction screens, was removed through dedicated quench circulation screens, situated below the extraction screens. This removed liquid was pumped through the central pipe into the middle of the digester above the extraction screens. From there, the liquor was supposed to flow towards the extraction screens to displace the more concentrated cooking liquor to reprocessing. This quench circulation flow, however, increased the amount of liquor flowing from the center of the digester towards the extraction screens, as well as its radial flow velocity, which increased the non-desired chip column packing against the extraction screens. The additional load due to the quench circulation worsened the operation of the screens in the loaded digesters and the movement of the chip column. Therefore, this design has been abandoned and the vacant quench circulation screens have in general been connected in parallel to the extraction screens to reduce the load of the latter. Due to the problems described above, circulations or extraction flows in the digester cannot be maintained at sufficiently high rates, which results in non-desired chemical and temperature gradients. This has a negative impact on the quality of the product and the production economy. In the worst case, the operation of the whole digester may stall due to screen blockages. Solutions for the problem have been presented for example in U.S. Patents Nos . 2,998,064; 3,385,753; and 6,129,816. The aim of these patents is to reduce the chip column compaction at the screen area by means of the screen construction itself, and simultaneously to prevent the screen from blocking and to thus improve the digester operation. Because the above-mentioned patents cannot repeal the laws of physics, i.e. the increase of chip column compaction against the screen caused by radial flow, the advantage of the methods disclosed in these patents is small and possibly non-existent. In U.S. Patent No. 3,475,271, a digester essentially for sawdust pulping is disclosed. In the immediate proximity of its bottom, in the middle of the digester, there is a rotating central screen, which is either cylindrical, or a cone the diameter of which increases in the plug flow direction of the chip column. Washing liquor to be added into the digester is pumped through a screen at the pressure vessel wall and flows horizontally towards the mentioned central screen, through which it is extracted to reprocessing. The aim of this arrangement is to carry out horizontal- displacement washing at the digester bottom. Physics causes problems in this design as well, as does the location of the screen in relation to the supply of wash liquid. The flow from the wall of the vessel towards its center encounters a decreasing flow area, which means increasing flow velocity. Consequently, the radial force vector towards the screen in the middle of the digester increases very quickly, even with small liquor flow rates (compared with flow in the opposite direction, from the middle towards the wall) . Because of this, a central screen according to U.S. Patent No. 3,475,271 blocks very easily. To be able to force the required amount of liquid from the distribution screen at the wall of the digester to the screen in the middle of the digester, both screens have to be very high in proportion to the digester volume, which makes this construction expensive. That is why this design has not been supplied commercially for decades . SUMMARY OF THE INVENTION In accordance with the present invention, these and other objects have now been accomplished by the development of a method for continuously producing pulp from lignocellulose-containing material in a digester including a top, a bottom, and a central pipe extending from the top to the bottom within the digester, at least one central screen assembly disposed on the central pipe, and a wall screen assembly disposed along the inner wall of the digester, the method comprising feeding a porous column of the lignocellulose-containing material from the top of the digester to the bottom of the digester, pumping liquor through the porous column of the lignocellulose-containing material in a predetermined direction, and simultaneously withdrawing the liquor through the at least one central screen assembly and the wall screen assembly. Preferably, the predetermined direction is concurrent, countercurrent, or a radial direction. In accordance with one embodiment of the method of the present invention, the wall screen assembly and the central screen assembly are disposed at substantially the same level within the digester. In accordance with another embodiment of the method of the present invention, the method includes withdrawing a plurality of separate streams of the liquor through the central screen assembly. Preferably, the central screen assembly comprises a plurality of distinct central screens. In accordance with another embodiment of the method of the present invention, the method includes recirculating at least a portion of the withdrawn liquor to the digester through a circulation loop. Preferably, the method includes heating or supplying chemicals to the recirculating portion of the withdrawn liquor in the circulation loop. In accordance with another embodiment of the method of the present invention, the digester includes a withdrawal screen disposed along the inner wall of the digester, and the method includes adding washing liquor to the digester adjacent to the withdrawal screen through the central pipe, whereby the washing liquor displaces the liquor through the withdrawal screen, and displacing at least a portion of the washing liquor through the central screen assembly by the washing liquor flowing countercurrent through the lignocellulose-containing material. In accordance with the present invention, these and other objects have also been accomplished by the development of a digester for continuously producing pulp from lignocellulose-containing material, the digester having a top and a bottom and comprising a top inlet for feeding the lignocellulose-containing material into the top of the digester, a bottom outlet for removing the cooked lignocellulose-containing material from the bottom of the digester, at least one wall screen assembly disposed along the inner wall of the digester for withdrawing liquor from the lignocellulose-containing material, a central pipe extending from the top to the bottom of the digester, and at least one annular central screen disposed on the central pipe for withdrawing liquor therethrough complementary with that of the at least one wall screen assembly. Preferably, the at least one annular central screen comprises a cylindrical central screen. In another embodiment, the at least one annular central screen comprises a conical central screen. In accordance with one embodiment of the digester of the present invention, the digester includes back-flushing means associated with the at least one annular central screen. In accordance with another embodiment of the digester of the present invention, the central pipe includes an inlet for introducing washing liquor into the digester, the inlet being disposed adjacent to the at least one wall screen assembly. According to the present invention, the excessive packing of chips in a moving column against an individual screen, and the zone of zero velocity in the middle of the digester, are avoided by using a screen assembly located on the central tube or pipe for liquor withdrawal together with a wall screen. The required outflow is thus divided between the wall screen and the central screen. In a preferred embodiment, the central screen is essentially at the same level as the wall screen. The central withdrawal screen structure in the middle of the pressure vessel may be located at a level different from that of the complementary wall screen assembly. In this case, it is so located that liquid flowing towards this central screen has to travel upwards through the chip column, and against the direction of the plug flow. This counter-flow reduces the chip column compaction against the central screen, especially at the bottom part of the screen, and the screen remains open and operational. With the screen solutions according to the present invention, the amount of circulation or extraction flows can be increased in proportion to the pulp production rate without increasing the load of an individual screen so much that it would disturb the movement of the chip column and have a negative influence on the quality and quantity of product. The present invention is applicable for all types of continuous digesters having various different circulation and extraction flow patterns, found on the market under a variety of commercial names . The present invention can be applied in continuous cooking both for the cooking vessel and the impregnation vessel. These vessels can be pressurized or un-pressurized. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is described in more detail with reference to the following detailed description, which, in turn, refers to the appended drawings, in which: Figure 1 is a side, elevational, partially schematic view showing the general operational principles of a typical single-vessel, hydraulic, continuous digester of the prior art; Figure 2 is a side, elevational, partially schematic view of one embodiment of the present invention, in which the extraction flow is divided into two parts, i.e. between the central screen in the middle of the pressure vessel and the screen structure at the wall of the pressure vessel; and Figure 3 is a side, elevational, partial, enlarged schematic view of a digester profile of the extraction screen region, and how the present invention is applied to improve the washing in the digester. DETAILED DESCRIPTION Figure 1 shows the general outline of a prior art continuous hydraulic digester. It consists of a pressure vessel (11) , upper and lower cooking circulation screens (12), mounted to the upper part of the pressure vessel (11) , extraction screens (13) mounted close to the middle part of the vessel, and optional washing circulation screens (14) mounted close to the bottom of the pressure vessel. Along the vertical center line of the vessel (11) there is typically a central pipe (15) , which comprises a single pipe or several nested pipes. Through this central pipe, separate circulation entry- flows are conveyed to different levels of the vessel (11) . General liquid flow directions inside the pressure vessel (11) toward the extraction screens (13) are shown by arrows (21) . The chips to be cooked and the liquid cooking chemical are introduced into the upper part of the pressure vessel (11) through a feed line (1) . In the pressure vessel (11) , the chips form a porous chip column, the porosity of which changes from the top down. The void space between the chips enables the flow of cooking liquor in desired directions inside the chip column as pressure differences are generated. The upper part of the pressure vessel (11) is typically an impregnation zone with a relatively low temperature. In this zone, the required cooking chemicals are impregnated into the chips . The chip column moving downwards and the cooking liquor flowing in its voids are heated by circulating this cooking liquor through the cooking circulation screens (12) using upper cooking circulation (2) and lower cooking circulation (3) . The upper cooking circulation (2) liquor is typically withdrawn from the upper screen (12A) and returned through the central piping at a position above that screen. Correspondingly, the lower circulation (3) liquor withdrawn from the lower screen (12B) is returned at a position in the screen zone as shown. The circulating liquors in the cooking circulation (2) and (3) are heated to the desired cooking temperature. Equipment required for pumping and heating the circulating liquor and/or addition of different chemicals and/or washing filtrate is not shown in figure 1. In the zone between the cooking circulation screens (12) and extraction screens (13) cooking reactions occur, resulting in softening of the chips. The gravitational force caused by the chip column height and the dynamic forces created by liquor flows, together with weakening of the mechanical strength of chips, result in increased compaction of the chip column, i.e. decrease of void space between the chips and of the relative proportion of voids in the digester volume. The maximum compaction of the chip column is typically at the extraction screen (13) , where the vertical velocity vectors of the cooking liquor (21) flowing downwards and the washing liquid (22) flowing upwards amount to zero. The maximum region of zero velocity (0) in the middle of the digester is also created at the same level. This region of zero velocity (0) weakens the efficiency of liquor displacements in the digester. Liquid consisting of cooking liquor (21) and displacing washing liquid (22), exits through the extraction screens (13) and is conveyed to reprocessing along line (4) . Counter-current washing of the cooked chips is carried out in the zone between the extraction screens (13) and the washing circulation screens (14) . Washing liquid, which has been pumped to the bottom part of the pressure vessel (11) through line (7), is first heated by means of washing circulation (5) to a desired value and re-enters the digester through the lower end of the central pipe. Subsequently, the upwards-flowing washing liquid displaces less pure cooking liquor towards the extraction screens (13) . Because the flow occurs by the action of pressure difference, the developing flow direction in the case shown in figure 1 is parallel to the straight line between the lower end of the central pipe and the extraction screens (13) . In the shaded region of zero velocity (0) in the middle of the digester, no direct displacement of liquors occurs. If the digester is not equipped with washing circulation (5) , this region of zero velocity is even larger than in the case shown. Figure 2 shows an embodiment in accordance with the present invention, in which a central screen (16) has been added to the central pipe (15) of the digester design according to figure 1. This screen is located essentially in the middle of the vessel (11) at the approximate level of the extraction screens (13) . The liquor extracted through the central screen (16) is conveyed through line (9) to reprocessing. A new flow channel can be installed for line (9) in the central pipe (15) , or existing flow channels can be used if such channels have possibly been left free following with some change, such as the removal of the above described quench circulation. The area of the central screen (16), mainly its height, is defined on the basis of the amount of liquor to be extracted through the central screen (16) . This extracted liquor amount is preferably nearly as large, and more preferably essentially equal to the liquor amount extracted through the extraction screens (13) . The position of the upper part of the central screen (16) in relation to the upper part of the extraction screens (13) affects the uniformity of the cooking result. In the case shown in figure 2, it is preferable that the upper part of the central screen (16) is above the upper edge of the extraction screen (13) , because there is always a higher temperature in the middle of the pressure vessel (11) than next to its wall. This temperature difference is due to heating circulations (e.g. circulation (2) and (3)) and heat losses through the pressure vessel wall (11) . The cooking reactions are terminated earlier in the central part of the pressure vessel (11) than at the periphery, as hot cooking liquor (21) is removed from the central part of the digester through the central screen (16) , and somewhat cooler cooking liquor (21) closer to the wall zone is withdrawn later through the extraction screen (13) . Cooking has then taken place at a slightly higher temperature in the middle of the pressure vessel (11) than at the wall, but during a correspondingly shorter period. This results in a uniform cooking result across the whole cross-section of the vessel (11) . The length of the central screen (16) is determined according to the capacity required. If a washing circulation (5) is used as shown in figure 2, the central screen (16) should not extend too near the inlet of the washing circulation in the middle of the digester, because there is a risk of short-cut flow from the washing circulation inlet directly to the central screen (16) . If a plurality of liquor ractions are to be extracted through the central screen (16) , it is divided into distinct sections with individual outlet channels, or a corresponding number of distinct screens is used. For example, cooking liquor (21) can be withdrawn through the upper part of the central screen (16) and washing liquor (22) through its lower part in desired proportions. Cooking and washing processes can thus be adjusted independently. If necessary, a blocked central screen (16) can be cleared by backflushing, in which liquor is momentarily pumped backwards at a high velocity. This back-flushing liquor can be brought backwards along line (9) , or a separate flow channel (not shown in the figure) with appropriate control equipment can be provided for it in association with the central pipe structure (15) . It is particularly advantageous to apply the present invention to a digester design having no washing circulation (14) in the bottom part of the pressure vessel (11), because washing liquid (22) in such digester designs has a particular tendency to channel, that is to flow directly from the entry point of the washing liquid (22) towards the extraction screens (13) . Figure 3 shows an embodiment of the present invention, in which improved washing is achieved in the extraction screens. In this embodiment, washing liquid (23), which is either washing circulation (14) liquid or fresh washing liquid from line (7), see Fig. 2, is supplied through a flow channel of the central pipe (15) , and enters the digester through a distributing section of the central pipe (15A) above the extraction screens (13) . This washing liquid (23) displaces the cooking liquor (21) from the chip column through the extraction screen (13) . This relatively cool washing liquid (23) terminates the cook effectively and evenly at the extraction screen (13) . Because washing liquor (23) is also cleaner than the bound cooking liquor inside the chips, diffusion starts immediately due to the concentration difference. The liquor and fibers inside the chips become cleaner, and the washing liquor (23) in the chip column voids carries the impurities. Diffusion time for the descending chip column is created by means of a non-screen part (15B) in the central pipe, located between the section for addition of washing liquor (23) in the central pipe (15A) and a central screen (15C) in accordance with the above disclosure. When the chip column descends towards the bottom part of the pressure vessel, impure washing liquor is displaced from the voids of the chip column by counter-currently rising washing liquor (22) . The displaced washing liquor (23) and part of the displacing washing liquor (22) are conveyed out of the vessel along a flow channel in the central pipe (15) . Using the disclosed method, pulp can be cooked and washed evenly. Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims .