separate load reduction mass by retained and treated

[austinorr]

current results report only mass from treated discharge, not mass reduced by the action of treatment. same for retention.

these values are computed for the mass-balance check, but not stored in the results output.

create new attrs f"{load_col}_removed_by_treatment" and f"{load_col}_removed_by_retention". check that removal by treatment + removal by retention = inflow load - discharge load