Reintroduce automatic free times

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