Optional dependencies for data downloaders

[davidwilby]

Relates to #106 and #102

• Adds some more detail to the user guide's data processor page and docstrings for two data downloaders on them needing optional dependencies (or more specifically, non PyPI compatible dependencies for get_station_data).
• Adds a check into get_earthenv_auxiliary_data that rioxarray is installed before downloading the data.
• For good measure, adds the sphinx todo exension for displaying todo directives in docstrings.

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[tom-andersson]

This is great, thanks @davidwilby! I wasn't aware of importlib.util.find_spec. With the get-station-data import check I just tried to catch an ImportError, but importlib is probably more principled.

One thing before LGTM, I realised the user will bump into this error in the Data Requirements notebook before the DataProcessor one. Would you mind moving your note to that page? For example, just after the Data Sources paragraph.

Also left a comment on the paragraph you added that needs resolving.

[davidwilby]

This is great, thanks @davidwilby! I wasn't aware of importlib.util.find_spec. With the get-station-data import check I just tried to catch an ImportError, but importlib is probably more principled.

That would be my usual approach but my thinking here was that since deepsensor doesn't import rioxarray directly (only via xarray), it would avoid a sort of needless import. But we end up just importing importlib instead, maybe this is just fancier :shrug:

Arguably, using try/except and catching ImportError is more usual and possibly more readable. But I'd rather import a standard library module and barely use it than import a third-party dependency and not use it at all, since it will be more liable to possible instability/weirdness in the future.

One thing before LGTM, I realised the user will bump into this error in the Data Requirements notebook before the DataProcessor one. Would you mind moving your note to that page? For example, just after the Data Sources paragraph.

Absolutely, will do!

Actually I've opted to include it in both sections since I think that it's possible that users will begin with the User Guide section. (See d3a5892)

Also left a comment on the paragraph you added that needs resolving.

No problem, agreed.

[tom-andersson]

LGTM, thanks very much for these doc improvements @davidwilby!