AMWG diagnostics variable enhancement request for "day-night" (dn)

[bertinia]

From @drmikemills: I would like to have the AMWG diagnostics updated so that these required variables:

'AODVIS','AODDUST','AODDUST1','AODDUST2','AODDUST3'

can be replaced with the "day-night" (dn) versions that are not masked to be missing at night:

'AODVISdn','AODDUSTdn','AODdnDUST1','AODdnDUST2','AODdnDUST3'

Perhaps this could be set up so that the "dn" versions are preferred, but the old (day-only) versions could be used if the dn variables are not present.

This could reduce the number of variables we output for CMIP6.