Add grads_dim to identifying criteria (#244)

Author: dcherian

cf_xarray/criteria.py

@@ -58,6 +58,7 @@
             "Pressure",
         ),
         "axis": ("Z",),
+        "grads_dim": ("z",),
     },
     "vertical": {
         "standard_name": (
@@ -77,15 +78,23 @@
         "standard_name": ("projection_x_coordinate",),
         "_CoordinateAxisType": ("GeoX",),
         "axis": ("X",),
+        "grads_dim": ("x",),
     },
     "Y": {
         "standard_name": ("projection_y_coordinate",),
         "_CoordinateAxisType": ("GeoY",),
         "axis": ("Y",),
+        "grads_dim": ("y",),
+    },
+    "T": {
+        "standard_name": ("time",),
+        "_CoordinateAxisType": ("Time",),
+        "axis": ("T",),
+        "grads_dim": ("t",),
     },
-    "T": {"standard_name": ("time",), "_CoordinateAxisType": ("Time",), "axis": ("T",)},
     "time": {
         "standard_name": ("time",),
+        "grads_dim": ("t",),
     },
 }