bug: cam-fv perfect_model has not been updated from CESM1_2 to CESM 2_1

[kdraeder]

Use case

Generate synthetic observations from CESM2_1 CAM6.

Is your feature request related to a problem?

There are no perfect_model scripts in cam-fv/shell_scripts/cesm2_1.

Describe the your prefered solution

Write a setup_pmo (modeled on setup_hybrid) and a perfect_model.csh (modeled on assimilate.csh).
Use any useful pieces from cam-old/shell_scripts/CESM1_2_1_setup_pmo and perfect_mode.csh.
I've done this and plan to submit a PR soon, unless someone objects.
It resulted in small changes to other cam-fv scripts and input.nml.

Describe any alternatives you have considered

There are none worth considering.

[kdraeder]

It appears that #233 (PR to fix this issue) was closed without merging the proposed fixes into main.
I don't know the reason. It may be because the discussion resulted in several proposals
for changes that seem outside the original scope of the PR, and one that's in the scope,
but that seems to require more discussion andor significant work;
rewriting the set up scripts to use a common resource file.
Then we got distracted, so this issue still needs to be fixed.