Move spherical harmonics to its own Appendix

[Lestropie]

The specification already has precedent for moving blocks of content to dedicated Appendix pages. I think that with the current detailed description of the spherical harmonics bases in use, it would make sense to move that content to a separate Appendix page. This would make it both potentially applicable to other modalities, and prevent it from interrupting the flow of the DWI models section quite so much.

[Lestropie]

I'm thinking that the definition of zonal spherical harmonics can also be made explicitly there.