Improve History Variable Metadata

### Issue Type

Infrastructure Update

### Issue Description

I think we should improve the descriptions of variables on the CAM history output. In particular the MAM descriptions remain quite opaque (I have realized this in trying to describe variables to students). It is something we have always threatened to do, so we should make the effort for CAM7.

As an example

--
float bc_a4(time, lev, lat, lon) ;
bc_a4:mdims = 1 ;
bc_a4:units = "kg/kg" ;
bc_a4:mixing_ratio = "dry" ;
bc_a4:long_name = "bc_a4 concentration" ;
bc_a4:cell_methods = "time: mean" ;

This has always seemed somewhat circular to me.
I nominate for some help
@cecilehannay
@tilmes
@fvitt

### Will this change answers?

No

### Will you be implementing this yourself?

Yes, but I will need some help