Implement unit testing

[jnfrye]

Currently, there are no unit tests, as the code is mostly composed of scripts. As I refactor and clean the code, I will add unit tests where I can.

When I add unit tests, I'm going to have module-specific tests placed in a <module name>_test.py file in the same folder as the module. Unit tests that involve multiple modules will go in the tests/ folder.

Here is the to-do list:

• [ ] Learn pytest
• [ ] Learn/try out hypothesis
• [ ] Add unit testing...
  • Put module-specific tests in files right next to the modules
  • Put multi-module tests in the project-level tests/ folder
• [ ] Add code coverage analysis