psf_photometry with masks, output as astropy.table.Table

**EXPERIMENTAL: DO NOT MERGE**

I started to add more features to the psf photometry (because I need it for a project). Unfortuntely, I did not finish it before I go on vacation, so I decided to put this half-working branch out there.

This is my plan for this PR:
- [x] psf photometry on the edge of the image
- [x] psf photometry with masks (particularly to make make useful fits with nans in the aperture)
- [ ] input and output with astropy tables
- [ ] apply the same changes to PRF fitting

So far, all development is in `GaussianPSF`, but the same applies to other PSFs and to PRFs, too. I think I can do PRFs in this PR, but later we will need some refactoring to make this work with arbitrary PSF models.

Comments on the API changes in here are welcome, other help is appriciated, too.