Compute Gaussian random field from power spectrum

[jasonmcewen]

We should have a function that takes a power spectrum and then compute a Gaussian random field with that power spectrum. While this is relatively straightforward, it would make things easier for users.

We already have functionality to compute Gaussian randoms fields on the sphere in signal_generator but this doesn't support a power spectrum. A good place for this functionality is probably in the same file.

[ntessore]

I'd be happy to give it a shot!