Set prescribed aerosol/aerosol deposition flux specifier based on prescribed_aero_model namelist value

[jimmielin]

From #331

In build-namelist in CAM, prescribed_aero_model is used to set the value for aerodep_flx_specifier and prescribed_aero_specifier based on whether its a bulk/modal aerosol model.

This functionality is currently not present in atmos_phys/CAM-SIMA and needs host-side modifications: as @nusbaume wrote

Namelist variables can be used to control other namelist variables in SIMA, it just takes some modifications on the CAM-SIMA side. You can see an example of this being done for the pver to nlev attribute setting in CAM-SIMA here:

https://github.com/ESCOMP/CAM-SIMA/blob/development/cime_config/buildnml#L338

This is an issue reminder to make such host model changes eventually to set the namelist values automatically, rather than setting manually in the namelist.xml files