Bug fix to failure of regional mean climate metrics

Changes are made to mean_climate_driver.py to call the updated version
of region_subset() function. Specifically, current mean_climate_driver.py
passed three input parameters in to region_subset() to mask the data for
regional mean climate calculation. However, the region_subset() in
pcmdi_metrics.io library required 5 input parameters, leading to the
failure of mean_climate_driver.py calculation.

pcmdi_metrics/mean_climate/mean_climate_driver.py

@@ -338,12 +338,18 @@
                                     ds_ref_dict[region] = ds_ref_tmp
                             else:
                                 ds_test_tmp = region_subset(
-                                    ds_test_tmp, regions_specs, region=region
+                                    ds_test_tmp,
+                                    region,
+                                    None,
+                                    regions_specs,
                                 )
                                 ds_test_dict[region] = ds_test_tmp
                                 if region not in list(ds_ref_dict.keys()):
                                     ds_ref_dict[region] = region_subset(
-                                        ds_ref_tmp, regions_specs, region=region
+                                        ds_ref_tmp,
+                                        region,
+                                        None,
+                                        regions_specs,
                                     )
                                 print("spatial subset done")