Add .gitignore for standard Python

[wmwv]

Add .gitignore for standard Python

Note: think carefully about FITS files. We do want some input, but not output. So perhaps include FITS (.fits, .fits.gz) in .gitignore and if anyone makes a change they'll have to remember to force the update.

@benjaminrose @sosey How do you feel about this list:
https://github.com/github/gitignore/blob/main/Python.gitignore

[benjaminrose]

I usually start with that repo and adjust as needed.

FITS: We can have predefined input names so we can add something like

*fits*
!input-*.fits

[sosey]

I would also add to ignore *.asdf files. We dont want to be committing data files, or very large files, unless there is an exceeding good reason, and even then there are limitations in github.

we can also assign a reviewer for any commits that include a change to .gitignore through a codeowners file