Simplify test converter add gromacs test

[oerc0122]

Description of work

• Refactor test_converter to reduce code duplication through pytest.mark.parametrize.
• Replace os.path with pathlib.Path
• Add gromacs-nvt test
  Instead implementing _converter_test to handle standard format testing.

Fixes
N/A

To test
Tests should pass.

[MBartkowiakSTFC]

One big advantage of the new version is that it is much shorter, and this for me counts already as 'easier to read'.

Would you like to replace the occurrences of the word typ with something clearer to a reader who expects it to be written in English?