Add unit testing for all major functions

[seananderson]

Now that the report has been reviewed and the chaos has subsided we really should make sure we have unit testing of all the major functions. Adding a coverage information badge with the covr would probably be a good incentive.

I'm not thinking of anything fancy. Primarily I think we just need to make sure that:

• the tidy and plot functions can then run on the output from the get data functions without an error
• the modelling functions run without an error