Add back namelists for: drydep, fire-emiss, and megan for the CESM2_3 branch, but only allow to be off

[ekluzek]

Add back in the atm/lnd joint namelists for: drydep, fire-emiss and megan, but only allow them to be off when coupled to SLIM. This will allow the model to tell you that you can't turn these features on when coupled to SLIM. CAM has a separate mechanism to turn them on, so we need to have SLIM check that they are turned off.