ReadTheDocs autoapi fixes

[kysolvik]

Fixes for #69

• Added Napoleon extension to support Google-style docstrings. Fixed most formatting, but need to go through to make sure everything is consistent and rendering correctly. Currently, there's a split between parameters and the docstrings for them. It might be cleaner if everything is shown together. Probably just an option I'm missing.
• Added automatic type-hint support via autodoc.
• Made dabench/data/* submodules non-public (_*) to match the rest of the package and to avoid confusing extra documentation of submodules that are only there for organization and aren't supposed to be imported directly.

Preview the new docs here: https://dataassimbench.readthedocs.io/en/docs-rtd-autoapi-fixes/autoapi/dabench/index.html

Will change to non-draft once I've checked through everything for consistency and looked into the parameter/docstring split problem mentioned above.

[kysolvik]

Found the fix for the parameter/docstring issue. Need to make sure we're distinguishing between attributes and args for classes. My fault for not being careful about that.

Good news is that this will fix a problem that I might not have found without this issue!

Bad news is I need to go through all classes and apply the fix.

Will update as soon as that's done.

[kysolvik]

@StevePny This is ready for review.

Solves #69.

You can preview the API reference docs here: https://dataassimbench.readthedocs.io/en/docs-rtd-autoapi-fixes/autoapi/dabench/index.html

I focused on docs updates so as not to change functionality in this PR, but there's some outstanding items with open issues #51 and #71