Add information to `calc_spherical_harmonics` docstring

[jamespanton93]

Pull Request Checklist. Please read and check each box with an X. Delete any part not applicable.

Updated the model.calc_spherical_harmonics docstring with information about convections used

For all pull requests:

• [X] I have run the contrib/utilities/indent script from the main directory to indent my code.

For new features/models or changes of existing features:

• [X] I have tested my new feature locally to ensure it is correct.

[jamespanton93]

Relating to issue #110