Erik/harmonicscomments

[je-fleischhauer]

This merge request builds upon #422 .

• My previous comments were a bit sloppy. I have revised the comments on the module. (But there is still room for improvement!)
• I adapted the unit test to increase the code coverage.
• I corrected a slip between order and degree of spherical harmonics.
• I added some checks to make the code more robust.

[fakufaku]

Thanks, this is great! I just found a couple typos in the docstrings. It would be great if you can fix them, and then I can merge it.

[fakufaku]

Thank you so much, this is great!