ZDC: update RPD calibration procedure

RPD analysis code allows for per-channel calibration factors to be applied during reconstruction. Previously, these factors have been included in checks for pileup to account for differences in noise between channels. This merge request restricts the scope of these calibration factors to use only for scaling the output of the RPD reconstruction (max and sum ADC values) and clarifies their use in documentation. In the future, separate per-channel calibration factors or thresholds may be needed to account for, e.g., differences in noise between channels.