implement fourier space moments from BFD

[beckermr]

@esheldon and I were thinking it'd be nice to have an implementation of the moments measurement in this paper: https://arxiv.org/pdf/1508.05655.pdf

I think this would be a nice addition to galsim in general, hence the issue.

[beckermr]

I'm happy to code this up if there is interest.

cc @rmjarvis @rearmstr @gbernstein @esheldon