Don't run radio process if there are no antennas
Right now, c8_airshower.cpp can run both CoREAS and ZHS formalisms. However, it might be that you only want to run one formalism without changing the cpp. If there are no antennas, then the ZHS and CoREAS formalisms should immediately quit. Right now, at least CoREAS does several opterations before hitting the for antenna
loop.