Use xgrid for aoflux calculation unless atm & ocn are on the same grid

Resolves #502

Author: billsacks

cime_config/namelist_definition_drv.xml

@@ -925,7 +925,10 @@
       default: ogrid
     </desc>
     <values>
-      <value>ogrid</value>
+      <value>xgrid</value>
+      <!-- xgrid introduces unnecessary cost and complexity if atm & ocn are on the same
+      grid, so fall back to ogrid in that case -->
+      <value samegrid_atm_ocn='true'>ogrid</value>
     </values>
   </entry>
   <entry id="ocn_surface_flux_scheme">