Adding small change to KSpaceSolver when -p_max flag is given on commandline.
Now outputs the time step (simulation time) that this happens, which allows for
saving displacement and refractive index data at only specific times when
the ultrasound has reached its focus.

Needs testing, and if it works, this time step should be saved to the output h5
by moddifying SaveScalarsToHDF5File() function and related files (e.g. TParameters).