Parse SEQRES record of PDB files for get_sequence()

[dnlbauer]

Sequence objects can be extracted from PDBx files. However, this is in theory also possible for many PDB files since they contain a SEQRES record, which also provides sequence information. Yet, The PDB parser does not offer this while the PDBx parser does.

It would be really helpful if the parser for PDB could support extracting sequences directly from the SEQRES record in PDB files and provide a get_sequence method.

[padix-key]

PDBFile has only quite limited capabilites compared to CIFFile/BinaryCIFFile as the format itself is deprecated. That being said, having PDBFile.get_sequence() would not add much complexity so I am in favor of adding this.

For now you could use structure.to_sequence(), that creates a sequence from AtomArray.