bug: too many indices in PlotClimatologicalSkillScore

  File "/opt/pycharm-community-2020.2.1/plugins/python-ce/helpers/pydev/_pydevd_bundle/pydevd_exec2.py", line 3, in Exec
    exec(exp, global_vars, local_vars)
  File "<input>", line 1, in <module>
  File "/home/l.leufen/mlair/venv/lib64/python3.6/site-packages/xarray/core/dataarray.py", line 196, in __getitem__
    labels = indexing.expanded_indexer(key, self.data_array.ndim)
  File "/home/l.leufen/mlair/venv/lib64/python3.6/site-packages/xarray/core/indexing.py", line 43, in expanded_indexer
    raise IndexError("too many indices")
IndexError: too many indices```

Was observed first on JUWELS, but is also reproducable on local machine:

* indexing is not suitable if only a single station is used!