Minor changes to prepare for merge

[grantfirl]

This PR has the following changes:

• change units of flags from none to flag to adhere to CCPP metadata guidelines: https://github.com/ESCOMP/CCPPStandardNames/blob/main/StandardNamesRules.rst
• remove dependency on module_mp_nssl_2mom in GFS_suite_interstitial -- passing in nssl_cccn and dividing by the reference density is preferable to always have to compile NSSL MP, even when not in use
• use assumed-shape arrays in mp_nssl.F90 to trigger array bounds checking and conform with CCPP guidelines