Fix SFFT stamp generator to not need FITS Images

[rknop]

See comment in image_subtraction.py::stampmaker.

The SFFT stamp generator routine is passed a FITS files, and reads something from the header. Eventually, we're not going to have FITS files, so we won't be able to get a FITS header that we can pass on to the stamp generator.

(This maybe should be done as part of a larger refactor to allow passing of image data to SFFT instead of FITS files, which will save us on I/O.)