Support Spherical Harmonics

This major release now supports the fast and backward stable transform between spherical harmonic expansions and bivariate Fourier series. Convert between representations as simply as:

julia> Pkg.add("FastTransforms")

julia> using FastTransforms

julia> F = sphrandn(Float64, 1024, 1024);

julia> G = sph2fourier(F; sketch = :none);
Pre-computing...100%|███████████████████████████████████████████| Time: 0:00:04

julia> H = fourier2sph(G; sketch = :none);
Pre-computing...100%|███████████████████████████████████████████| Time: 0:00:04

julia> norm(F-H)
1.1612440912382192e-12