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']`.
Merge pull request #147 from hamogu/patch-2

Update psf.py
Add segment_photometry
Update segment_photometry docs
Allow label subset for segment_photometry
Change index to labels and make flux nan for invalid labels
Implement error, gain, and mask options in segment_photometry
Implement background option in segment_photometry
Add mask interpolation
WIP: segment_props
Rename segmentation module
WIP: segment_props
WIP: segmentation props
Clean up segment_photometry api docs
Move centroid, area, etc. from segment_photometry to segment_props; more clean up
More work on segment_photometry
Add background to output table
Fix gain issues
Fix error issues
Add background columns to docs