Update psf.py

We should set the fixed / not-fixed here in every call, since we don't know what state they are in before. Sure, the psf object is initialized with those values fixed, but they may have been changed in a first run of `psf_photometry`. Let's say I don't like the result and run `psf_photometry` again with `tune_coordinates=False` then I want them to be fixed, independent of the current value of `psf.fixed['x_0']`.