RNPxl: also use PD-style protein accession formatting

[hannesveit]

Use the PD notation, e.g., "P20053" instead of "sp|P20053|PRP4_YEAST" (see issue #8)

Unfortunately, the workaround from issue #8 doesn't work here, as PD does not perform the search on its own and hence TargetProteins is empty. It is unclear what the exact parsing rules are that PD internally applies to the "original" accession strings from the FASTA file.

Thus, for now it is probably safer to stick with the original FASTA accessions.