Add GHA tests forcing formatting

[eirikurj]

Purpose

Avoid merging any incorrectly formatted bib entries.
To do that, this PR adds a formatting check with GHA running bibtool on push to master and PRs. The bibtool resource file is also renamed, both to keep it hidden and bibtool will search for it.

Expected time until merged

No rush

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[ewu63]

Don't have the powers anymore but LGTM