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

This operator allows the computation of the kinetic energy related to a
body with spatial Inertia I and spatial velocity v

Revised the unitests, added some doc and created a "check" compilation rule. All unitests passed ok.

Minor correction to improve computation time in Sym3. (error in the commit: unittest symmetric working, but tspatial is broken. Fixed in the following commit.)