Increase performance in LCT model loop

[mknaranja]

@lenaploetzke just pointed me to the question if we could not extract the part (FP)LctStateGroup::template get_num_subcompartments<InfectionState::InfectedSevere>() * (1 / params.template get<TimeInfectedSevere<FP>>()[Group]) from the loop in

memilio/cpp/models/lct_secir/model.h

Lines 256 to 257 in 54aa7db

	flow = (FP)LctStateGroup::template get_num_subcompartments<InfectionState::InfectedSevere>() *
	(1 / params.template get<TimeInfectedSevere<FP>>()[Group]) * y[ISevi_first_index + subcomp];

to avoid redundant computations and to improve performance. This is probably a minor change and could be tested quickly if there is some spare time.

These also appear multiple times.