PEPTIDE and PROPEP (CHAIN, maybe even SIGNAL-peptides) may have an unique identifier

[Luxxii]

Currently these features (except for CHAINs), currently do not provide these IDs in FASTA_Headers. We should expand this!

It may be enough to change the following lines:

(SIGNAL)
https://github.com/mpc-bioinformatics/ProtGraph/blob/master/protgraph/export/peptides/pep_fasta.py#L103

(PEPTIDE)
https://github.com/mpc-bioinformatics/ProtGraph/blob/master/protgraph/export/peptides/pep_fasta.py#L116

(PROPEP)
https://github.com/mpc-bioinformatics/ProtGraph/blob/master/protgraph/export/peptides/pep_fasta.py#L112