Is your feature request related to a problem? Please describe.

Currently namelist.py provides the complete structure for a model namelist, including the physics settings. This means that

1. as we develop the Physics we need to also create a branch of NDSL to use in parallel,
2. the namelist is tailored to specific models instead of providing a general platform, and
3. the namelist in NDSL is getting pretty bloated as more physics is integrated.

This is not a sustainable way to develop a model and breaks the concept of NDSL as a middleware platform.

Describe the solution you'd like

NDSL should provide the infrastructure to create namelist configs for model components like a NamelistFactory and then provide that for the dycore. Then the physics schemes like pySHiELD can import NamelistFactory to create PhysicsConfig. Pace itself can then import these and compose them into a single expected namelist.

Describe alternatives you've considered

Alternative ideas are welcome