Add some unit capabilities to make_* functions
Merge pull request #174 from larrybradley/datasets_units

Add some unit capabilities to make_* functions
Add functions to generate Gaussian example images
Add tests for Gaussian example images
Merge pull request #175 from larrybradley/new_datasets

Add two new images to datasets
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