Adding p_grating keyword to NIRSpec Dark/Superbias datamodels

[stscijgbot-jp]

Issue JP-3904 was created on JIRA by Christian Hayes:

NIRSpec dark and superbias selection criteria on CRDS now include grating information, so delivering darks requires a P_GRATIN keyword to be set.  We would like to have the NIRSpec dark/superbias datamodels updated (although I believe these are pulling from the generic dark/superbias models) to include the p_grating keyword so it can be set automatically in reference file generation (which uses jwst datamodels).

The one caveat is that old files do not have this keyword set.  I believe that the datamodels gracefully handle missing keywords, but I wanted to double check that ahead of making any changes.