add support for getting the best-fit model parameters

[lgarrison]

Astropy Lomb Scargle has a model_parameters() method that returns the least-squares fit at a single frequency. I don't think we need to use finufft for this, but it would probably make sense to add to the nifty-ls API. In the meantime, users can use nifty-ls for the periodogram and Astropy for the model parameters.

Thanks to @soichiro-hattori for the recommendation!