Make sense of & elaborate on nMOLDYN-related comments

**Issue description**
The test files `test_FQt.py` and `test_SQw_gaussian.py` compare MDMC observable calculation to nMOLDYN data. These tests contain a variety of workaround and monkeypatches (like setting the oxygen scattering length to 0, or using hardcoded values instead of calculating them to match nMOLDYN's slightly different values). These comments are frustratingly vague and it seems we've forgotten what they were about.

**Suggested fix**
The assigned developer (preferably with nMOLDYN knowledge) should look into these test files, figure out why we do the workarounds, see if they are all necessary, and add extra comments to explain why they are done with technical detail and values for each software.