Return footprint and added accuracy tests

@mdboom - this returns the footprint as you suggested - I decided to just return it by default

@bluescarni - I added some basic accuracy tests between the different modes and compared to montage. I also made a very small change in the C that may change the values a little (just in case it matters for the montage comparison). Will add a note below.
@bluescarni - here it turns out the original code had a mistake because one should be finding the ratio of the intersection to the final area in order to find the fractional contribution to the final pixel.