Implement systematic spec preprocessing in base class and as util functions #318
Description
The table tennis environment implements a preprocessing function that procedurally adapts the the base MJCF asset for the environment based on the config of the env. This is a useful and versatile approach, and could be added to the base class and inheriting envs can decide to implement it. Useful procedural scene editing functions such as removing dofs or mirroring/scaling bodies and chains could be generalized in helper utils.