Flat-field step doesn't handle ref file DQ flags properly

[hbushouse]

The Build 6 testers in INS have discovered that the DQ flags from a flat-field ref file are not being handled properly within the flat-field step. This is due to the fact that the flat-field ref file - at least in some cases - is being loaded into a MultiSlitModel and this data model does NOT call the routine to perform dynamic DQ flag remapping. So the ref file DQ values are being left as is, instead of being translated via the settings in the DQ_DEF table included in the ref file.

A couple of possible solutions include:

1. switching the flat-field step back to loading ref files using a FlatModel, now that we don't have NIRSpec flats in the form of MultiSlitModel's anymore. The FlatModel data model does apply the dynamic DQ remapping.

2. Adding dynamic DQ remapping to all data models (such as MultiSlitModel), at least when a DQ_DEF table is available.

[hbushouse]

Fixed in #404