Edit doc of cubic_spline, exact_cubic, quintic_spline, spline_deriv_constraint. Modify name of variables min and max to t_min and t_max.

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

Use point_t::Zero(Dim) instead of point_t::Zero()  because it is required to use a waypoint_t as templated type