Update astropy_helpers
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
More work on segment_props