faces_dataset_to_latlon function requires metric_vector_pairs to be present

[rabernat]

The default value of metric_vector_pairs in faces_dataset_to_latlon is metric_vector_pairs=[('dxC', 'dyC'), ('dyG', 'dxG')]. The function errors if I call it on a dataset that doesn't have these variables present.

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~/Code/xarray/xarray/core/dataset.py in _construct_dataarray(self, name)
    975         try:
--> 976             variable = self._variables[name]
    977         except KeyError:

KeyError: 'dxC'

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-11-dc713ebaf709> in <module>()
      1 ds = xr.merge([sst, u, v])
----> 2 ds = llcreader.llcmodel.faces_dataset_to_latlon(ds)#, metric_vector_pairs=[])
      3 ds

~/Code/xmitgcm/xmitgcm/llcreader/llcmodel.py in faces_dataset_to_latlon(ds, metric_vector_pairs)
    289         data_vars[vname_v] = xr.Variable(_drop_facedim(ds[vname_v].dims), data_v, ds[vname_v].attrs)
    290     for vname_u, vname_v in metric_vector_pairs:
--> 291         data_u, data_v = _faces_to_latlon_vector(ds[vname_u].data, ds[vname_v].data, metric=True)
    292         data_vars[vname_u] = xr.Variable(_drop_facedim(ds[vname_u].dims), data_u, ds[vname_u].attrs)
    293         data_vars[vname_v] = xr.Variable(_drop_facedim(ds[vname_v].dims), data_v, ds[vname_v].attrs)

~/Code/xarray/xarray/core/dataset.py in __getitem__(self, key)
   1056 
   1057         if hashable(key):
-> 1058             return self._construct_dataarray(key)
   1059         else:
   1060             return self._copy_listed(np.asarray(key))

~/Code/xarray/xarray/core/dataset.py in _construct_dataarray(self, name)
    977         except KeyError:
    978             _, name, variable = _get_virtual_variable(
--> 979                 self._variables, name, self._level_coords, self.dims)
    980 
    981         needed_dims = set(variable.dims)

~/Code/xarray/xarray/core/dataset.py in _get_virtual_variable(variables, key, level_vars, dim_sizes)
     80         ref_var = dim_var.to_index_variable().get_level_variable(ref_name)
     81     else:
---> 82         ref_var = variables[ref_name]
     83 
     84     if var_name is None:

KeyError: 'dxC'

Workaround for now is to just pass metric_vector_pairs=[], but this is obviously a bug.