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

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

The gain is about 10% for NLE and RNEA.

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

[Major][C++] Renamed macro for spatial traits : 2 cases -> use in templated/not templated classes to avoid polluting warnings. Following these naming could avoid multiple definitions of macro SE3_JOINT_TYPEDEF

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