handle gravity as an acceleration

[sbarthelemy]

Currently, gravity is handled as an additional force, in a dedicated controller. It could be computed more efficiently as an acceleration offset, acting on a FreeJointWithGravity joint.

[salini]

it seems that the WeightController has some problem: for example, the compute H_bc is not used, and the inverted Adjoint, is not transposed, as it is multiply with a "force".