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The new algorithm computes also data.J to efficiently stored the
previous oSk motion space.

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

[C++] Add com information in data + compute position, velocity and acceleration of CoM in the same loop + add related unittest

There is still an issue on ConstraintXd.
This commit *does not* compile is here for book-keeping.