Fix hangs in ocean conservation

A recent change switched to using xarray to concatinate ocean
conservation data together.  However, recent testing showed hangs
if the datasets are not manually loaded.  This merge adds 2 places
where such manual loads are included.

Author: xylar

mpas_analysis/ocean/conservation.py

@@ -769,6 +769,9 @@ def _compute_time_series_with_xarray(self, variable_list):
         unique_indices = sorted(unique_indices)  # Ensure ascending order
         ds = ds.isel(Time=unique_indices)
 
+        # seeing hanging during saving.  Let's try loading
+        ds.load()
+
         if append:
             # Load the existing dataset and combine it with the new dataset
             self.logger.info(
@@ -785,6 +788,9 @@ def _compute_time_series_with_xarray(self, variable_list):
         self.logger.info('Sorting by xtime...')
         ds = ds.sortby('xtime')
 
+        # again, seeing hanging during saving.  Let's try loading
+        ds.load()
+
         # Save the resulting dataset to the output file
         self.logger.info(
             f'Saving concatenated dataset to {self.outputFile}...')