Clean-up after UC9 merge

[JanisGailis]

• Use @op_return instead of @op_output
• Change 'filter_dataset' to 'select_variables'
• Use @op with tags
• Use 'ds' as the default name for xarray datasets
• Ensure operations take CLI passable inputs, e.g., use single values where possible
• Don't use the 'required' keyword
• Update the 'timeseries' operation so that it checks that the time dimension is there and that data is aggregated along other dimensions.
• Try to make slow tests faster.