compute_derivate return raw pointer instead of shared_ptr (to use covariant return type, required for python API)

Modif for spline deriv constraint to exact cubic : delete spline_deriv_constraint.h and modifications in other classes and tests

Fix evaluation of bezier of degree 0 added, Edit doc on bezier_curve, Edit assert_allclose to assertTrue in test.py

change namespace spline to curve in include folder, rename folder spline to curve, change in the CmakeList.txt

added waypoints at zero velocity in quaternion interpolation. Added copy constructors for exact_cubic and curve_splines. template type for exact_cubic primitive splines