Change path to volcanic aerosol data

The volcanic aerosol data was moved to a new location within the
GOCART emissions directory.

Refs: #1592

parm/chem/SU2G_instance_SU.rc

@@ -8,7 +8,7 @@ aerosol_monochromatic_optics_file: ExtData/monochromatic/optics_SU.v1_3.nc
 nbins: 4
 
 # Volcanic pointwise sources
-volcano_srcfilen: ExtData/volcanic/so2_volcanic_emissions_Carns.%y4%m2%d2.rc
+volcano_srcfilen: ExtData/nexus/VOLCANO/v2021-09/%y4/%m2/so2_volcanic_emissions_Carns.%y4%m2%d2.rc
 
 # Heights [m] of LTO, CDS and CRS aviation emissions layers
 aviation_vertical_layers: 0.0 100.0 9.0e3 10.0e3