Add netherlands data

[joseph-palmer]

This PR adds a class for the Netherlands, with data provided by the RVIM (English: National Institute for Public Health and the Environment). Data includes new cases, deaths and hospital admissions at the province (level 1) and municipality levels (level 2).

The source of the data is a single csv file which has the data at the municipality level and so the class has a clean_level_1() function which aggregates the data at the province level.

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[seabbs]

LGTM - nice stuff!

[seabbs]

Looking at the CRAN logs looks like it doesn't