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

[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

[Major] spatial class SE3 is now CRTP. Move traits class in spatial/fwd.h as it is now used in spatial classes and joint-base

Warning remaining at compilation of CRBA.
Timings:
nq = 36
RNEA = 		4.79756 us
CRBA = 		5.50824 us
Cholesky = 	2.40469 us
Jacobian = 	3.68493 us
COM+Jcom = 	4.024 us