Implement unit testing

[ryantibs]

Now that we have some collaborative software development happening thanks to @matteo-fontana @paolo-vergo and team, we should really implement unit tests in this package. We can use the testthat package, for example.

Paolo and team, can you guys start doing this at least for the new stuff you're adding? (Ideally we'd write tests to cover everything that's there as well---if you wanted to do that, then this would be excellent.)