Reintroduce automatic free times

[PierreMartinon]

keyword free instead of numeric value for initial/final time in .def (or if too annoying use additional keys such as free.initial.time true/false)

Internally:

• use [0,1] fixed time grid
• append parameters for free times, with small lower bounds
• rescale time from [0,1] to [t0,tf] for OCP functions and time steps in dynamics equations, as well as solution file
• note that this changes the type of the final_time and time arguments passed to the OCP functions, so the declarations in each problem.cpp file need to be updated -_- It would be more practical to have this block included from a single source file in src/, if possible

[PierreMartinon]

Started the free final time case. Initial time is way less frequent so will probably be postponed, maybe to the full event times handling.
Tests:

• fixed final time case should not be affected
• free final time case (not minimum time) eg goddard
• minimum time case eg ?