Dynamically determine major species

### Issue Type

Infrastructure Update

### Issue Description

@fvitt suggested that the model could dynamically determine what the major species are, thus taking the onus off of the namelist.

That said, the default namelist settings do set the major species correctly and the namelist option enables users to perform tests (remove some major species, etc).

One option would be to have the model, by default, determine the active species but have an optional namelist override that can be used for testing. Not sure if that would cause more confusion than necessary

@gold2718 @cacraigucar @nusbaume

### Will this change answers?

No

### Will you be implementing this yourself?

No