Added methods in joints and algorithm working on Model to determine if two configurations are equivalent

[C++] The method forwardKinematics now selects by itself the section of the right DoFs over a full vector

This new method give the increment for finite difference applications.

Most of the joints have bugs inside their methods - integration,
differentiate, interpolate.
A small remind that the joint velocity vector is generally expressed in
the joint frame while the translations are expressed in the joint
parent frame.

It replaces the old shortname  method with is no more static

Those assignments lead to infinite recursion loops

[C++] Factorize code with JointRevolute for JointRevoluteUnbounded types such as MotionRevolute(Unbounded), Constraint

* Rename Scalar_t to Scalar.
* This allows applying Eigen-compatible template operations to
  se3::spatial objects.

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

Conflicts:

	CMakeLists.txt

[C++] The stack of joints model & data is now of type joint accessor ( i.e the derived types are encapsulated in a generic joint accessor

Conflicts:

	src/multibody/model.hpp

With next version of Eigen3, it seems that there exists a missing
initialiser when calling the cast operator.

Eigen::ProcuctReturnType becomes Eigen::Product in Eigen version >=
3.3.0.

[C++][FixBug] Fix the visitor for calc (0 and 1st order) and calc_aba + added visitor on the shortname of a jointmodel derived type