Update diag table for coupled aerosols

Now that aerosols are permitted to run with coupled, a new diag table
is needed for that configuration. Instead of adding nested ifs to
handle the different combinations, the coupled diag table is moved
out into its own block, then if aerosols are on, '_aer' is appended to
the file name, whichever diag table is used.

In the future, I hope to break diag_table up into pieces that can be
assembled based on component (see Issue NOAA-EMC#567).

Refs: NOAA-EMC#516

parm/config/config.fcst

@@ -307,14 +307,17 @@ elif [[ "$CDUMP" == "gfs" ]] ; then # GFS cycle specific parameters
 
 fi
 
+if [[ $DO_COUPLED = "YES" ]] ; then # coupled model
+    export DIAG_TABLE="$HOMEgfs/parm/parm_fv3diag/diag_table_cpl"
+fi
+
 if [ $DO_AERO = "YES" ]; then # temporary settings for aerosol coupling
-    export DIAG_TABLE="$HOMEgfs/parm/parm_fv3diag/diag_table_aer"
+    export DIAG_TABLE="${DIAG_TABLE}_aer"
     export FIELD_TABLE="$HOMEgfs/parm/parm_fv3diag/field_table_aer"
+    # TODO
     export CHM_CFGDIR="not_set"
     export CHM_INPDIR="not_set"
     export dnats="$(( $dnats + 0 ))"
-elif [[ $DO_COUPLED = "YES" ]] ; then # coupled model
-    export DIAG_TABLE="$HOMEgfs/parm/parm_fv3diag/diag_table_cpl"
 fi
 
 # Remember config.efcs will over-ride these values for ensemble forecasts