This PR extends theflatsys module to include two new elements:

• flatsys.BSplineFamily adds B-splines as a new type of basis for solving trajectory generation problems (withinflatsys oroptimal).

• flatsys.solve_flat_ocp() allows solution of optimal control problems for differentially flat systems with trajectory and terminal costs and constraints, mirroring the functionality ofoptimal.solve_ocp().

In the process of making these additions, a few other changes:

• Theflatsys.BSplineFamily introduces the ability to have multi-variable basis functions (where each flat output uses a different basis function from a selected family). This is useful if you have a differentially flat system in which you would like to have different degrees for different variables.
• Fixed a bug in the computation of derivatives for the Bezier family of basis functions with rescaled final time, and implemented a final time rescaling for the polynomial family of basis functions.
• Added a more descriptive__repr__ for basis functions (show the family + information on attributes).
• New documentation (sphinx + docstring) and units tests for the added functionality.