Better preview images for WATA files

[melanieclarke]

NIRSpec WATA files have an unusual raw data format, which makes the rate files produced by the stage 1 pipeline useless to look at. The TA source can sometimes be seen in the preview image for the uncal files, but sometimes it has to be checked in the explore image link, by differencing the uncal group 0-1 and 1-2. This makes reviewing the WATA images inefficient, when usually all that's needed is a glance to determine that the TA image looks okay.

It would be nice to generate a better default preview image for WATA files, so that they can be reviewed at a glance.