GitLab

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

It also computes:
- Jcom directly (works also for robot without FF)
- Energetic terms

Use the name for all the algo related to centerOfMass computations. All
of them now return the center of mass position of the whole rigid body
system. Add also to compute only the position and velocity of the CoM

The idea is to separate the real spatial acceleration of a body to the
spatial acceleration containing the gravity field, which is useful in
the context of dynamical equations

[C++]{Minor] Changed the name of the method to access the segment of a given vector. The length of the returned segment depends on the dof of the joint(size NQ or NV)

[C++][Major] Reorganized the methods to acces segments of fixed size (one for NQ, one for NV) from joints indexes