[C++][unittest] Fix computations in jointcomposite::calc() + reworked unittest (comparison of R3xSO3 (either two joints with no inertia between them or Jointcomposite))

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++][Unittest] Reworked JointAccessor Unittest. tests are now done on every type of joint of the boost variant Model

Completed the visitors on JointVariants for each method of the JointBase interface  and moved it to a different file.