RT: allow flexible number of sub-cycles determined by local conditions

Main part of this MR is to relax the use of TimeIntegration:max_nr_rt_sub_cycles from being (ab)used as a fixed number of sub-cycles each particle does, to actually being the maximal number of permitted RT sub-cycles per hydro step. The conditions are now being set by the local radiation and gas conditions. If a particle's hydro time step exceeds the maximal permitted number of sub-cycles, then the hydro time step is reduced accordingly.

The "debug" RT scheme doesn't have any physics, so it doesn't have a way to determine a proper RT time step size. There, the use of TimeIntegration:max_nr_rt_sub_cycles remains as before - it determines the exact number of sub-cycles for each particle. This way, the scheme remains useful for developing and debugging purposes.

Since RT and hydro are independent now, the same "neighbor time-bin limiter" has been added for RT time bins.

Additionally, I added some minor fixes for GEAR-RT (enable correct restarts, some additional checks...)