Incorporate tests

[dchud]

@eedrummer had a start on these on the refactor branch. Pull them in and identify more opportunities to test - even if there isn't a good testing story on chart generation.

[dchud]

As part of this, consider consolidating tests under a tests dir and moving the python module files into a growthviz dir. Imports would then need to change accordingly e.g., to from growthviz import sumstats. Probably a good idea given the number of code files we now have.

[dchud]

Per discussion: let's focus this ticket on software-level checks, as opposed to user-supplied data checks. Our main goal with these is to verify that our library functions return as expected with the sample data and perhaps some perturbed/extreme subsets of that sample data.

Please use python's built-in unittest library. Ultimately we should end up with a tests/ directory at the top level, with one or more python test scripts inside that directory that can be called directly or discovered automatically by running python -m unittest at the top level of the package.

Some brief tutorials that might help (let me know if you're looking for more!):

• https://www.freecodecamp.org/news/an-introduction-to-testing-in-python/
• https://pythontesting.net/framework/unittest/unittest-introduction/