Add _resize_resampled_images method to MosaicResampler

Despite giving Swarp a target FITS frame to resample into, it may still
produce frames with linear shifts in CRPIXi and in NAXISi. This method
takes the resampled images, and the target FITS image, and pads/trims
the resampled images so that NAXISi and CRPIXi match.