Export Dataset to CSV

[Tianchi-Liu]

This PR allows users to export xarray Dataset to the CSV format. Previously only DataArray can be exported to CSV. Now, users are able to export station data as well as all the Dataset outputs from the threshold_tools_basics and threshold_tools_application_examples notebooks.

Implementation details:

• The function export_dataset no longer raises an exception when users export a Dataset. The Dataset would be exported via _export_to_csv.
• The functions _export_to_csvand _metadata_to_file are branched depending on whether the data to export is a DataArray or a Dataset.
• Helper functions are created/updated to hold code that works for both data types.
• Existing code specific to DataArray is moved to _dataarray_to_dataframe with structural changes needed to leverage the helpers.

As an example, for a Dataset containing 3 stations, the header of the exported CSV looks like this:

Note: The climate variable name is not currently in the header as it is not readily available in the Dataset. Will be pulled from variable_description.csv later.

To test: In the refactor branch of cae-notebooks, try export some station data and/or other Dataset.

[elehmer]

Thanks @Tianchi-Liu! I have merged this into the refactor branch. I renamed export_dataset to export and exposed it as a top level method. The Export class has been removed along with the dialog