Don't set vary_shift in calc_interact.
Don't set vary_shift in calc_interact.

This allows the vary_shift_from option to be used - and the shift will still
start varying from that iteration anyway, as a negative target must be less
than the current population.

hande-qmc/hande,hande-qmc/hande,ruthfranklin/hande,hande-qmc/hande,hande-qmc/hande,hande-qmc/hande