Add option relax_terminal_physical_time for time_freezing problems

[nurkanovic]

We have this for the plain numerical time, to make an ell_1 relaxation of the terminal time constraint.
It would be nice to have the same also for physical time constraints (maybe even also for the stagewise constraints - but to be discussed).

I want also to test with this the ease of use of vdx, once we migrated to it.