ROCKER ARM ASSEMBLY FOR AN ENGINE TECHNICAL FIELD This invention relates to a rocker cradle and rocker arm assembly for an engine, such as a rocker cradle and rocker arm assembly used in the valve train of an internal combustion engine. BACKGROUND In a typical valve-in-head engine, a rocker arm is used to drive the valves, by pivoting under the influence of the pushrods. Fig. 1 shows such a typical engine 30. The engine 30 is of a V-8 type, and has an engine block 32 housing a crankshaft 34 which is driven by pistons 36 in the well- known manner of a combustion engine. The crankshaft also drives a cam shaft 38 bearing integral cams along its length. These cams are arranged to actuate components of the valve train in order to operate the intake and exhaust cycles of the valves associated with each piston. Each cam 38 engages a cam follower 40 which, in turn, connects to a pushrod 42 extending between the cam follower 40 and a rocker arm 44. The rocker arm 44 is pivotably mounted on a rocker cradle 50; and actuates a valve 46 against the bias of a spring 48 when the pushrod 42 moves toward the rocker arm 44, as the rotary action of the cam 38 is transferred through the cam follower 40. In a V-8 engine, four opposed sets of valve train components are located on either side of the cam shaft; but many other engine designs use pushrods in the same or a similar manner. Typically, the rocker arm 44 and rocker cradle 50 are attached to the engine cover by bolting the rocker cradle 50 to the engine cover; or by bolting a U-shaped clamp down over the base of the rocker cradle 50, thereby holding the rocker cradle 50 down. In either case, the rocker arm 44 is typically mounted on the rocker cradle 50 by a bearing; and the rocker cradle 50 is attached to the engine cover using a bolt. Also, each rocker arm is mounted on a separate rocker cradle 50. There is an ongoing need, however, for lighter, stronger, and more convenient rocker arm assemblies. SUMMARY In one embodiment according to the invention, there is disclosed a rocker cradle for an engine, the rocker cradle being integrally-formed and comprising a plurality of hoops for supporting at least one rocker arm pivotably mounted on a spindle, the plurality of hoops being formed integrally with the rocker cradle. In further related embodiments, the rocker cradle may be capable of accepting the installation of the at least one rocker arm while at least one valve of the engine is depressed using a valve-compression tool. Also, the rocker cradle may be capable of accepting the installation of the at least one rocker arm without a bolt being secured against the bias of a valve spring. The rocker cradle may comprise high-quality steel, or a metal-matrix composite material. In another embodiment according to the invention, there is disclosed a rocker arm assembly for an engine. The rocker arm assembly comprises a rocker cradle for an engine, the rocker cradle being integrally-formed and comprising a plurality of hoops for supporting at least one rocker arm pivotably mounted on a spindle, the plurality of hoops being formed integrally with the rocker cradle; and at least one rocker arm pivotably mounted on a spindle supported on at least one of the plurality of hoops. In further related embodiments, each of the at least one rocker arms may comprise a reduced mass overhang. The spindle may comprise high-quality steel with a lubricating coating, and the at least one rocker arm may comprise high-quality steel. Four rocker arms may be pivotably mounted on the integrally-formed rocker cradle. The engine may be a V-8 engine; and four rocker arms may pivot against four valves of the V-8 engine. The engine may comprise at least one corresponding pair of pushrods and valves, between each corresponding pair of which one of the at least one rocker aπns is pivotably mounted. In another embodiment according to the invention, there is disclosed a method for installing a rocker arm of an engine. The method comprises using a valve- compression tool to depress at least one valve of the engine; placing the rocker arm in an integrally-formed rocker cradle; passing a spindle through a hoop in the integrally- formed rocker cradle and through the rocker arm; and releasing the depressed valve onto the rocker arm. In a further embodiment according to the invention, there is disclosed a method for removing a rocker arm of an engine. The method comprises using a valve-compression tool to depress at least one valve of the engine; removing a spindle from the rocker arm and from a hoop in an integrally-formed rocker cradle; and removing the rocker arm from the integrally-formed rocker cradle. Additional advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and accompanying drawings or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and as to how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: Fig. 1 is a cross-section through a typical valve-in-head combustion engine; Fig. 2 shows a three-dimensional view of a rocker arm assembly, according to an embodiment of the invention. DETAILED DESCRIPTION Fig. 2 shows a three-dimensional view of a rocker arm assembly 200, according to an embodiment of the invention, in which the assembly 200 may be lighter, stronger, and more convenient than conventional rocker arm assemblies, and have less inertia in the rocker arms. In the embodiment of Fig. 2, a set of rocker arms 201-204 are mounted on a single integrally-formed rocker cradle 205. The rocker arms 201-204 pivot upon a set of spindles 206-209, which are inserted through a set of hoops, such as hoop 218, formed integrally with the rocker cradle 205. Under the influence of a set of pushrods, indicated by dashed lines 210-213, the rocker arms 201-204 pivot to drive a set of valves 214-217 (valve 215 being indicated by a dashed line). There are a number of advantages of the embodiment of Fig. 2. First, because the rocker cradle 205 is integrally-formed, it is stiffer than the separate rocker cradles 50 of Fig. 1, and therefore provides better mechanical support under the duress of use. Next, using a single integrally-formed rocker cradle 205 removes the need to bolt and/or clamp each rocker cradle to the engine cover, as is done with the rocker cradles 50 of Fig. 1. The arrangement of Fig. 2 is lighter, because it removes the need for bolts and/or clamps as in the rocker cradles 50 of Fig. 1. In addition, it is more convenient, because it allows the rocker arms 201-204 to be installed or removed while valves 214-217 are depressed using a valve-compression tool. In particular, whereas the rocker arms 44 of Fig. 1 are conventionally installed by bolting and/or clamping the rocker cradle 50 to the engine cover against the bias of the valve springs 48, the rocker arms 201-204 of Fig. 2 can be installed or removed while the valves 214-217 are depressed by a valve-compression tool. To install the rocker arms 201- 204, the valve-compression tool is used to depress the valves 214-217; the rocker arms 201-204 are placed in the rocker cradle 205; the spindles 206-209 are inserted; and the valves 214-217 are released onto the rocker arms 201-204. Similarly, to remove the rocker arms 201-204, the valve-compression tool is used to depress the valves 214-217; the spindles 206-209 are removed; and the rocker arms 201-204 are removed. Thus, the embodiment of Fig. 2 alleviates the need to bolt down the rocker cradle against the bias of the valve springs, which had previously been considered necessary. Furthermore, the embodiment of Fig. 2 has the advantage of removing the need for using a bearing between each rocker 44 and its cradle 50 (as is done in the engine of Fig. 1), because each rocker arm 201-204 is mounted on a spindle 206-209. Also, in an embodiment according to the invention, the mass of the ends of the rocker arms 201-204 may be reduced, for instance by shortening the length of the arms and streamlining their shape. This reduced mass overhang of the rocker arms 201-204 reduces the pivot inertia of the arms, so that less force is required to drive the pushrods, thereby speeding up the engine. As shown in the embodiment of Fig. 2 for a V-8 engine, a single rocker cradle 205 may support rocker arms 201-204 for half of the valves of an engine - i.e., here, four rocker arms are supported by rocker cradle 205, for four valves of the V-8 engine. Another rocker cradle 205 could be used for the other four valves of the V-8 engine. It will be appreciated that an appropriate number of integrally-formed rocker cradles 205 may be used to support rocker arms for each of the valves of an engine, whether it has eight valves or any other number of valves; and that rocker cradles could differ from engine to engine in terms of the number of rocker arms used on each rocker cradle 205. In accordance with an embodiment of the invention, the rocker cradle 205 may be made of high-quality steel, such as EN16 T-condition; or a metal-matrix composite (MMC) material. Also, the spindles 206-209 may be made of high-quality steel, such as Aubert and Duval BMV4, with an appropriate lubricating coating; and the rocker arms 201-204 may be made of high-quality steel, such as Aubert and Duval BMV4W. Those of skill in the art will appreciate that a variety of suitable materials may be used for various components, in accordance with an embodiment of the invention; including (without limiting to this list), steel or metal alloys, carbon fibre materials, and ceramic materials. Also, the shapes of components shown herein should not be taken to be limiting; instead, a variety of different forms may be used for the various components, including the rocker arms, rocker cradle, spindles, and other components. Embodiments according to the invention may also be used in a variety of different engines in which rocker arms are used, and need not be limited to V-8 engines. Those of skill in the art will also appreciate that while the foregoing has described what is considered to be the best mode and, where appropriate, other modes of performing the invention, the invention should not be limited to the specific configurations and methods disclosed in this description of the preferred embodiment. Those skilled in the art will recognise that the invention has a broad range of applications in many different types of machinery using pivoting arms, and that the embodiments may take a wide range of modifications without departing from the inventive concept. For example, the invention has applications in all manner of combustion engines, for example, engines in road vehicles, specialist vehicles, ships and trains; but will have particular application where a light, strong, and conveniently- accessed rocker assembly is important or useful.