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.

[github-actions[bot]]

👋 Thanks for opening this pull request! Can you please run through the following checklist before requesting review (ticking as complete or if not relevant).

• [x] Read our contribution guidelines if you have not already done so.
  • [x] If you have altered an existing class please run the tests locally (using devtools::load_all(); devtools::test()) first setting options(testDownload=TRUE, testSource=class-name) and report your findings.
  • [x] If you have added a new data class please run the tests locally for that class (using devtools::load_all(); devtools::test()).
  • [ ] Check your code passes our CI checks and review any style and code coverage warnings.
  • [ ] Comment with details if unable to get our CI checks to pass or unable to remove all warnings.

Thank you again for the contribution. If making large scale changes consider using our pre-commit hooks (see the contributing guide) to more easily comply with our guidelines.

[seabbs]

Looking at the CRAN logs looks like it doesn't