JWST headers in preprocessing

[gbrammer]

These fixes apply updates to the JWST headers to make them look like HST for AstroDrizzle, to and fro a few times where various functions need it. This should fix #69.

Basically, everything that needs to be done to the JWST images for preprocessing is done in prep.fresh_flt_files, which calls out to jwst_utils.initialize_jwst_image for JWST exposures.

This includes:

• Update headers with HST-like keywords, making copies of the original keywords
• Apply the flat-field correction with the pipeline step if necessary
• Scale by the gain if BUNIT=DN/S
• Update wcs headers with the pipeline step, including making grism exposures "look like" direct exposures so that the 2D WCS gets populated
• Clip maximum DQ bits to 16 bit integers

A few other modifications include

• The hard-coded reference filenames have also been replaced with calls to the pipeline steps.
• Don't do "fix_star_centers", which was clipping out the centers of bright cluster galaxies