Renamed JointXXAccessor to JointXX + renamed traits<D>::JointData to traits<D>::JointDataDerived, same for Model.

Conflicts:

	CMakeLists.txt
	src/algorithm/center-of-mass.hxx
	src/multibody/model.hpp
	unittest/CMakeLists.txt

[C++] bodies are now a subpart of the Frames : some Frames are of type BODY and when referring to getBodyId/Name in Model it will look in its frames

[C++] Make distinction between joint and body in Model API ( addJoint, appendBodyToJoint, addJointAndBody instead of addBody)

[C++][FixBug][Unittest] Fix bug in calc_aba (bug with boost:fusion preventing a proper acces to jmodel.axis) + reworked unittest for joint prismatic unaligned

[C++][FixBug][Unittest] Reworked unittests for joint revolute unaligned + Fix bug in calc_aba (bug with boost:fusion preventing a proper acces to jmodel.axis)

  * se3::Model::getBodyId was used to retrieve joint id in parsers.
  * Implement new methods to retrieve joint ids.
  * Fix body methods.

[Major] Correction of bugs in JointRevoluteUnaligned and its unittest. The urdf parser nows takes into account this joint. Added a new enum in cartesian axis