Fix treatment of energy cut settings in ProposalProcessBase.hpp

With the PR !345 (merged) we introduced an energy_production_threshold. For PROPOSAL, this means that only stochastic energy losses about this energy are produced.

Currently, PROPOSAL still uses 50% of this value as a threshold. This is still a remnant from back when we had no differentiation between energy_production_threshold and energy_propagation_threshold. Now, this 50% value makes no sense anymore.