make `fre.cmor.cmor_mixer` more configurable and less hard-code dependent

[ilaflott]

will be closed by picking up #227 and running with it

[ilaflott]

closing as this is superseded by #232. That issue contains a sample of a way to gut cmor_mixer's hard-code and put it elsewhere. I'm not convinced this is worth acting on yet, and i'm not convinced the amount of hard-coded parameters is the primary source of pain- it's the gigantic problem space.