Added tests and minor changes to psf.py

This PR inlcudes a few more tests for psf photometry and a few minor changes to `psf.py`. I renamed the `flux` parameter of the `GaussianPSF` to `amplitude`, which makes it consistent with the `DiscretePRF` and `astropy.modeling`.  `subtract_psf`subtracted from the data array itsself, instead of working on a copy of the data. This has changed now.  Furthermore I renamed the `eval` function to `evaluate`, to follow the changes in `astropy.modeling`. 
@adonath Doesn't the function normalization need to be changed when switching from `flux` to `amplitude`, or was `flux` really just "amplitude" and not really the total integrated flux (as the docs state)?