add custom collective transforms

[CosmoMatt]

@jasonmcewen this PR builds on the Fourier-Wigner precompute transform work and implements custom transforms which collectively transform:

• Subsets of Wigner coefficients (collection of spin spherical harmonic coefficients) to collection of spin signals on sphere
• Collection of spin signals on sphere to their spin spherical harmonic coefficients (stable wrt spin and can accept duplicated spins as per multimodal data)
• Signal on SO(3) on a subset of its Wigner coefficients (subset of spin spherical harmonic coefficients).

These are all the core transforms we will need going forward.

[jasonmcewen]

Looking good @CosmoMatt, just a few minor comments. Have approved so you can merge once these addressed.