Better circle edges in photometry

[rknop]

Right now (ass of PR 213), the aperture phtoometry class makes a circle by including all pxiels whose center is within the circle. We need to weight the pixels around the edge based on the intersection of the pixel square and the circle.

One possibility is the shapely library; I haven't used this before, so I don't know it's the right answer, but it looks promising:

>>> circle = shapely.Point( [ 10.2, 10.5 ] ).buffer( 3.2 )
>>> square = shapely.Polygon( [ (11.5, 7.5), (12.5, 7.5), (12.5, 8.5), (11.5, 8.5) ] )
>>> circle.intersection(square).area
0.6191305416936494
>>> square.area
1.0