Description
This is an initial step in porting a Fortran shallow water test to pace. Rossby-Haurwitz wave 4 test case found in tools/test_cases.F90 (shallow-water test 6) of the GFDL_atmos_cubed_sphere repo. This flag does nothing as of now, but it will eventually be used to

This PR will add a sw_dynamics flag default value to NamelistDefaults. This change is the result of the following PR: NOAA-GFDL/pyFV3#50

Fixes # (issue)
None

How Has This Been Tested?
This has been tested locally. It will eventually be tested more formally via this related PR: NOAA-GFDL/pace#112

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - I don't believe there is corresponding documentation for this change.
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included - This will eventually be tested via the following related PR: [On-hold] Rossby Test Case Initialization + Example Config pace#112