Implement the following SpanSet operations:

• Construct from an ellipse - note geom::ellipses::PixelRegion; this should do most of the work.
• Compute centroid - see old Footprint implementation
• Compute shape (quadrupole moments) - see old Footprint implementation

One complication here is that this will introduce a circular dependency between afw::geom and afw::geom::ellipses. That's easy to address at the C++ level, but it's tricky in Python (which package imports the other?) I'll be emailing dm-devel shortly to start a discussion on how to address this problem.