This commit allows a cost function and constraints to be passed to theflatsys point_to_point() function and when present will use sp.optimize tofind a trajectory.  Preliminary unit tests, benchmarks, docstrings anddocumentation also in place.Switched the order of arguments in point_to_point() so that Tf (or timepts)now comes before the initial and final states, consistent with orderingelsewhere in the package.Make some small updates to optimal.py docstrings and argument names to makethings consistent with flatsys.

This commit changes the way that cost functions and constraints are handledfor flat system to carry out optimization only in the null space of the flatsystem basis coefficients, eliminating the use of an equality constraint forthe terminal condition.