Setting namelist defaults with multiple physics suites?

There are multiple CAMDEN namelist options whose default values depend on the physics option (`phys`) chosen. Normally this could be taken directly from the `physics-suites` config option, except currently the code allows for multiple physics suites to be listed even when there is only one model instance.

What should be done for the namelist defaults in the case of multiple physics suites but only a single host model instance? I can see a few possibilities:

1. Simply disallow situations where the number of physics suites listed is greater than the number of instances (`NINST`), and assume each instance only has one associated physics suite.

2. Have `buildnml` automatically choose a particular physics suite as the namelist default (e.g. the first suite listed in `--physics-suites`).

3. Have the namelist defaults depend on the specific compset chosen (e.g. `FKESSLER`) instead of the physics suites listed.

Any thoughts or opinions? To be honest this might tie into a bigger question of what to do with multiple physics suites that conflict with each other in several different ways, such as containing schemes that represent the exact same physical processes (which could result in either double-counting or one scheme never running, both of which seem bad).