New appendix page: spherical harmonics

[Lestropie]

Closes #100.

I'm not 100% happy with the structure: there's a section on "truncated spherical harmonics", and there there are tables relating maximal spherical harmonic degree to number of coefficients, yet the former don't reside in the latter. But they can't just be moved because of other order dependencies.

[Lestropie]

Excess """ characters at ends of equations were in the code prior to this review, so some copy-and-paste error was made upstream.

[arokem]

Excess """ characters at ends of equations were in the code prior to this review, so some copy-and-paste error was made upstream.

That's probably something I did 😄