Testing the concept of having a separate aperture class for SkyCircularAperture
Fixes
Updated tests
Partially fix skycoord test
Fixed some doctests
Added missing import
Fix remaining failures in building docs
Move skycoord_to_pixel to its own file
Added docstring to classes
Correctly treat radii in angle units
Use r in arcseconds in documentation
Minor cleanup in SkyCircularAperture
Tidy up parametrization
Address comments by @larrybradley, and removed option to pass sky coordinates without SkyCoord
Fixed commas
Added API reference
Remove pixelcoord argument
Updated front page of docs, and improved plot() so that it can take a source id or list of source ids
Added ``source_id`` to all plot methods
Updated aperture photometry docs