Write unit tests and run with CI/CD pipeline

RapidPE has many callable functions, and none of them have unit tests. Ideally we should have all functions unit-tested, but for this current issue the goal is to set up the unit testing framework, integrate it with the CI/CD pipeline, and write a few small unit tests to make sure it works.

• Set up unit testing framework (PyTest?)
• Write a few small unit tests as a starting point
• Integrate unit testing with CI/CD pipeline

After this issue is completed, we should create separate issues for each module that needs unit testing.