Convert energy_production_threshold value from kinetic energy to total energy

The EnergyCutSettings of PROPOSAL are set via the set_energy_production_threshold(p_code, energy) parameter. (I am not really happy about this solution, since wrong settings [or even just using the default settings] for this parameter can lead to nonphysical/misleading results).

Nevertheless, with the current implementation, the emCut parameter that is passed to cross_builder (and is called, for example, from here) is a kinetic energy (as all energies in CORSIKA 8 are, if I understand correctly).

However, PROPOSAL always expects total energies. This is why we need to add the mass to the emCut energy before passing it to PROPOSAL.