Added `copy_extensions()` and `combine_fits_extensions()` which correctly copies the extra image extensions from the input chips, as well as sets sensible values for blank/grid images. Cloes #31.
Added `copy_extensions()` and `combine_fits_extensions()` which correctly copies the extra image extensions from the input chips, as well as sets sensible values for blank/grid images. Cloes #31.

sweverett/Balrog-GalSim,sweverett/Balrog-GalSim