use scale_back=False so that images have BSCALE/BZERO applied..  Needed for IRAF.

We are moving to a unified location for pulling images from storage. Previously we didn't scale the data on read,
(JJ thought we did but scale_back=True means 'don't scale back' which is odd)
Now uses the storage.get_image function instead of downloaders private version.
remove loading the discovery images first.

Seamed like a good idea at the time.  OK, still could be but the current implementation is flawed.  Sometimes the
2 images that come after discovery are NOT the discovery triplet members.  This could be reimplemented using the
is_discovery marker to find all the members of the discovery triplet... maybe later.
Corrected spelling error on ccdnum.