Copy photutils.utils functions
Update imports for new functions
Replace fix_prf_nan with mask_to_mirrored_num
Add tests for mask_to_mirrored_num
Add mask_to_mirrored_num to __all__
Update mask_to_mirrored_sum docs
Add example for mask_to_mirrored_num
Merge pull request #11 from larrybradley/photutils

Add photutils.utils functions