Modify get available datasets

[joseph-palmer]

Rather than initialising every class when calling get_available_datasets this PR moves the data produced by get_available_datasets to package data (contained in data/) called all_country_data (happy to change this name). Now calling get_available_datasets() returns the saved data (and also filters for type using the type argument). The previous version of get_available_data() is now in a new function called render_available_data() which, when called will create the table by initalising each class like the previous version did.

A new R file called render_available_datasets.R in data-raw/ writes the all_country_data to the package data (which is also defined in datasets.R). To ensure contributors run this after adding a new dataset class I have added a test which will compare the current available datasets with that outputed by running render_available_datasets(). This will fail if a dataset class is added or if something in the class which is used in the table is modified (such as number of levels or urls) but the all_country_data table has not be re-rendered by running the render_available_datasets.R script. If this fails it will highlight this and suggest to run render_available_datasets.R.

closes #372

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[seabbs]

ah did we include this in the news and increment the dev version?

[joseph-palmer]

Ah no I forgot, sorry!

[seabbs]

No problem we can slip it in as a news update prior to release.