Integrate JHU and Google data sources

[joseph-palmer]

WIP

This PR will integrate JHU and google data sources into the package (most likely in a later release).

TODO:

• [x] complete JHU (#228) and merge into this branch.
• [ ] complete Google data (#229) and merge into this branch.
• [ ] Modify existing get_national/regional_data functions to handle these new sources.
• [ ] upload to master with a new release.

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[seabbs]

Just to note (and as you may have seen) I changed the country field to origin after some feedback. This will need changing here (sorry) but I think is actually quite nice given this is adding lots more none country based datasets.