Spherical Harmonic bugs/enhancements

This issue is a placeholder for spherical harmonic bugs/enhancements that will be part of a patch:

• the way the butterfly algorithm is currently implemented, it only works for dimensions that are powers of two, though nothing stops this from working for general dimensions. This afflicts FastSphericalHarmonicPlan and ThinSphericalHarmonicPlan. For the time being, it's recommended to just pad with zeros to the nextpow2.

• the commands sph2fourier and fourier2sph should work on different band limits in l and m.

• the command plan_sph2fourier is not type-stable as it very crudely dispatches to a different SphericalHarmonicPlan based on the bandlimit. This is negligible compared to pre-computation costs for large degrees (and is somewhat likened to Base's factorize).

• the convenience constructors sphrand, sphrandn, etc... take m and n and create an array with dimensions m and 2n-1 which is arguably misleading.

• it would be great to have an allocation-free FFTW override for converting the bivariate Fourier array to function values on the sphere at tensor product grids equispaced in angle. (Added starting with 67a6b560735ef8f82e1c6a7f82dad3e1a843fc8e)

• remove segmentation fault when using an even number of columns, thanks to @meggart for finding this. (Added starting with c0e4a546c3b7507efa44b905fb5bafa6379e20e0)

• add methods for complex data,

• and, NUFFT variants for nonuniform point.

Start by reviewing plan_sph2fourier and the remaining checklist items in this issue. The unresolved work covers type stability, complex-data methods, and NUFFT variants for nonuniform points; done means those requested spherical-harmonic capabilities are implemented and validated.