Fix .isel and .sel with dictionaries (#54)

Author: dcherian

cf_xarray/accessor.py

@@ -276,6 +276,7 @@ def _get_measure(da: Union[xr.DataArray, xr.Dataset], key: str) -> List[Optional
 #       We need this for groupby("T.month") and groupby("latitude") for example.
 _DEFAULT_KEY_MAPPERS: Mapping[str, Mapper] = {
     "dim": _get_axis_coord,
+    "indexers": _get_axis_coord,  # sel, isel
     "dims_or_levels": _get_axis_coord,  # reset_index
     "coord": _get_axis_coord_single,
     "group": _get_axis_coord_single,

cf_xarray/tests/test_accessor.py

@@ -154,6 +154,11 @@ def test_kwargs_methods(obj):
         actual = obj.cf.isel(T=slice(2))
     assert_identical(expected, actual)
 
+    with raise_if_dask_computes():
+        expected = obj.isel({"time": slice(2)})
+        actual = obj.cf.isel({"T": slice(2)})
+    assert_identical(expected, actual)
+
 
 def test_kwargs_expand_key_to_multiple_keys():