Extend search-query package with parser for PubMed search queries

[k-schnickmann]

This pull request extends the search-query package with a PubMed parser, enabling the input and syntactic validation of PubMed search queries.

Main Changes

• Implemented the PubmedParser class to support the parsing of PubMed queries.
• Added validation methods for syntactic validation of PubMed queries.
• Added custom PubMed error codes and support for search field translation in constants.py.
• Defined custom PubMed exceptions in exception.py.

[k-schnickmann]

Hi @geritwagner, I’ve found another small error in the PubMed parser: at lines 244 and 341, compound queries are incorrectly getting a field value assigned. Both of those should be set to None. See commit for fixes.

[geritwagner]

Thank you for catching the error and suggesting the fix. This is probably something I introduced when rewriting the code. I just integrated it here.