Add covid19 data hub wrapper

[joseph-palmer]

This PR adds a Covid19DataHub class which pulls and cleans data from Covid19 Data hub, specifically files marked as raw data (https://covid19datahub.io/articles/data.html). rawdata-1, rawdata-2 and rawdata-3 get level 1 (country) level 2 (region) and level 3 (subregion) data respectively.

The data collected runs through our generic cleaning and processing steps (of course with cleaning selecting and aligning the correct columns for this process).

The names of regions perhapse need changing and region codes are a little odd. ATM iso_alpha_3 is used for country, iso_level_2 is used for region and iso_numeric is used for subregions. Perhapse we could link these with the codes Google uses in their dataset?

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