Add testing infrastructure

[athowes]

I think I can look at some of the unit tests in e.g. epinowcast and start doing similar for the functions we have in epidist. Only worry is that I might do tests for something we end up getting rid of. I have some sense of what seems core to start with though.

Agree. I think the order should be

• Testing infra
• Cull functions (Which models to keep?  #15)
• Test what is left
• Features and robustness improvements

Looking at epinowcast might be a good shout or well done packages like dplyr

Originally posted by @seabbs in #5 (comment)