Edit polynom to polynomial in all files. File polynom.h edited to polynomial.h and bezier_polynom_conversion.h to bezier_polynomial_conversion.h

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.

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

"Unit tests" not seriously done yet (just string outputs) , should use mockpp or something