Handle Proteome Discoverer scores

[524D]

It would be nice if mzRecal could use mzid data produced by Proteome Discoverer, since many datasets in PRIDE contain search results in that format. However, the PD score reporting is in a non-standard userParam tag, where the meaning of score value is unknown. E.g.

        <SpectrumIdentificationResult spectraData_ref="XXX" spectrumID="scan=1234 file=678" id="SIR_1234_678">
          <SpectrumIdentificationItem passThreshold="true" rank="1" peptide_ref="-9876_56432_00000" experimentalMassToCharge="666.666" calculatedMassToCharge="666.777" chargeState="3" id="SII__1234_44444_1">
            <PeptideEvidenceRef peptideEvidence_ref="PE_44444_-1111" />
            <userParam name="Proteome Discoverer Delta Score" value="0.7070" />
          </SpectrumIdentificationItem>
          <cvParam accession="MS:1000894" cvRef="PSI-MS" name="retention time" value="99.9999" />
        </SpectrumIdentificationResult>