Fix PUM/UIM filter error when not applying TDCFs

This fixes the failure that occurs when running gstlal_compute_strain in full calibration mode ("unified," "DCS," "C01") without applying the TDCFs. The fact that the new unified filters files produced by pydarm do not contain a combined PUM/UIM filter was causing this failure. Now the filter is computed as the sum of the PUM and UIM filters.