Implement particle-track substepping for radio-emission calculation

The radio-emission simulation needs very fine stepping. Reducing the "maxRadians" value for the maximum allowed deflection angle makes the code slow, because all modules are then called very often, even the ones that would not need to be called in such a fine-grained way (e.g., hadronic interactions).

One could instead keep a relatively coarse general stepping, but do sub-stepping only within the radio module. This would have the further advantage that the sub-steps would be consecutive, i.e., one could re-use the propagation solution for the previous endpoint for the next startpoint.