Add automated unit testing

[OrangeJuice7]

• [ ] Implement unit tests
• [ ] Unit tests should automatically run on PR
• [ ] Implement code coverage
• [ ] Add build passing/failing badge to readme
• [ ] Implement some coding style enforcement

[OrangeJuice7]

Will use

• [ ] CI: Github Actions or Travis
• [ ] Building: CMake
• [ ] Testing: Catch2 https://github.com/catchorg/Catch2/blob/master/docs/tutorial.md
• [ ] Static analysis: cppcheck
• [ ] Coding style checker: cpplint
• [ ] Code coverage: gcov
• [ ] Code profiling: gprof
• [ ] Documentation (optional): Doxygen

See https://lefticus.gitbooks.io/cpp-best-practices/content/02-Use_the_Tools_Available.html

Also check out https://arne-mertz.de/2017/04/continuous-integration-travis-ci/