Significant cleanup of geometry code, implemented Cython code for subpixel mode for elliptical apertures
Whitespace
Further simplify logic in functions for aperture photometry
Simplify how annulus photometry is handled
Fix doctest
adding do_rectangular_photometry function
adding do_rectangular_photometry usage to aperture_core
minor docs change
adding rectangular annulus aperture
remove check for zero lenght aperture side
Adding test
removing repeated lines
Fixing code and tests to pass tests
Simplifying error calculation to use find_fluxvar()
creating rectangular_overlap() function to simplify do_rectangular_photometry(); and updating RectangularAnnulus.do_photometry