Store the scaling factor of the WFSS background subtraction step

[stscijgbot-jp]

Issue JP-3808 was created on JIRA by Gael Noirot:

The scaling factor between the WFSS background models and the science images in the background subtraction step is not stored presently. This is a useful value for users who might want to check the scaling on their end, and apply corrections/changes. It could be stored as a new keyword in the fits files.

[stscijgbot-jp]

Comment by David Law on JIRA:

Seems reasonable to me; as a strawman proposal I'd suggest BKGD_SCL as a floating point value to record the scaling factor.  This currently isn't used by any other observing mode, but should be general enough to do so if in future that proves necessary.