Modified use of ds.mean() to overcome dask error:

- Error was: NotImplementedError: Computing the mean of an array containing
  cftime.datetime objects is not yet implemented on dask arrays
- Solution was to force loading of the Dataset, with `ds.load().mean()`
- See: #185

clisops/core/average.py

@@ -176,6 +176,11 @@ def average_over_dims(
 
     # The mean will be carried out on a Dataset or DataArray
     # Calculate the mean, skip missing values and retain original attributes
-    ds_averaged_over_dims = ds.mean(dim=dims_to_average, skipna=True, keep_attrs=True)
+
+    # Short-term solution to error: "NotImplementedError: Computing the mean of an " ...
+    #    "array containing cftime.datetime objects is not yet implemented on dask arrays."
+    # See GITHUB ISSUE: https://github.com/roocs/clisops/issues/185
+    # The fix is simply to force `ds.load()` before processing
+    ds_averaged_over_dims = ds.load().mean(dim=dims_to_average, skipna=True, keep_attrs=True)
 
     return ds_averaged_over_dims