Unit testing

**Is your feature request related to a problem? Please describe.**
There is no testing, beyond use cases from elsewhere.

**Describe the solution you'd like**
Add some unit tests, preferably to the CI (which currently only does a build test).

**Describe alternatives you've considered**
Try to unit test `irt` in EICrecon, but that requires knowledge of the geometry or some sample simulation data. To test smaller pieces of the code, it would be better to do some unit testing here.

Note that in the ePIC software stack, we also have `reconstruction_benchmarks`, which test the full PID (currently just the dRICH).

**Additional context**
This is motivated by `Catch2` testing from EICrecon; since this `irt` repository is now used in EICrecon, it would be a good idea to do some unit testing here.