Array API: port the Gaussian angular power spectrum functions #488

The functions that compute Gaussian angular power spectra for the transformed (e.g. lognormal) fields is currently implemented for NumPy only.

I have started to port the underlying Fast Legendre Transform package flt so that it supports generic Array API backends. This is currently still on the main branch at https://github.com/ntessore/flt but will hopefully have a release soon. It will always require a case-by-case implementation due to the fft.dct/fft.idct calls, but both NumPy and JAX are supported already.