Clean up keyword n_avg

[rlbyrne]

The keyword n_avg need a default reported in the dictionary. It seems to be used in both gridding and Healpix outputs and has different defaults in different parts of the code. We need to clean up, document, and possibly rename the keyword.

[bhazelton]

Agreed. I find it particularly confusing that there is another keyword, nfreq_avg which means something totally different but sounds very similar every time I see it.

[mwilensky768]

@bhazelton not that it solves this issue, but I'm working on a PR that just changes nfreq_avg to beam_nfreq_avg which is slightly more illustrative.

[bhazelton]

That will help a lot, thanks!