lvl3 mthds: resp.pc + pvalzero

[Kelly-Carstens-EPA]

For endpoints where lvl3 methods are assigned 'resp.pc' and 'pvalzero', the response value will flip in the opposite direction from the lvl0 values, based on the logic:
resp.pc = (cval - bval)/ (pval - bval) * 100

For example: If bval is 100 and cval for the treatment well is 20 (this is a decreasing effect), the control well resp.pc would be 0% and the treatment well resp.pc would be 80% (this would plot as an increase).

This logic was based on old tcpl schema when we wanted to flip the fitting direction for bidirectional endpoints so the curve was always fit in the increasing direction. For the new schema, we need to use lvl3 mthd 'resp.multneg1' for endpoints where lvl3 methods are resp.pc and pvalzero. Suggest integrating a step where, if lvl3 mthd 'pvalzero' and 'resp.pc' is applied, we multiple the resp value by zero. This improves efficiency so 'resp.multneg1' doesn't need to be applied in order for the curve to reflect the biological direction from lvl0 data.