Huge commit!

1. now the bounding box is exclusively inside the physics engine. There isn't anymore duplication inside the embodied entity.

2. removed the <arena_physics> section of the XML. The assignment is made automatically.

3. lots of doxygen documentation added everywhere.

TODO: the <distribute> tag now does not work anymore, needs a fix.
Merge branch 'master' of https://github.com/ilpincy/argos3