Manually set Swarp resample output shape
Define PA as true PA, theta as rotation angle
resample propagates background uncertainty through skyoffset
Finally got the rotation math right
Hook up flag_paths with skyoffset resampler
Add set_unweighted_pixels_to_nan to resample

This forces unweighted pixels (due to flag masking) to have a value of
NaN, and thus be easily splotted when measuring photometry in the bins.
Fix bad typo in wedge measure_image sigma calc