Cleanup sphere averaging and integration

[kburns]

A few simple cleanups are needed on averaging and integrating for spheres:

• [ ] Averaging on the SphereBasis currently returns a SphereBasis with shape (1,1) instead of a constant.
• [ ] Averaging only works with S2Coordinates.
• [ ] No integration over the SphereBasis is defined.

Reference mailing list post: https://groups.google.com/g/dedalus-users/c/zIj-rBvvp68/m/RwhlZaEsFAAJ

[dhrichards]

It would be great if this was implemented