Refactored the dynamics 2d engine to reflect the structure in the PhysX engine.

This new structure has much better code reuse.

TODO:
- fix the foot-bot
Merge branch 'multibody' of https://github.com/ilpincy/argos3 into multibody
Augmented the logic of CDynamics2DSingleBodyObjectModel::SetBody().

This change makes it a little better to setup the body of an object.
The method now contains logic that would be repeated over and over anyway.