Add unit test framework

[cmovic]

Hi, I'm Vic. I'd like to contribute to the fplan project but I'm deathly afraid of breaking something. I think adding a unit test framework and some simple tests is a start toward building up a robust test environment where I and others will feel more comfortable making changes and having some knowledge that the wheels aren't coming off the train.

I've got experience with pytest so that's an easy place to start. I'm open to other testing frameworks but hope to keep this change simple.

I'm proposing that we create a test top level directory and that it mirrors src and looks something like this:

$ ls -R src test
src:
fplan

src/fplan:
fplan.py

test:
fplan

test/fplan:
test_load_file

test/fplan/test_load_file:
test1.toml  test_load_file.py

pytest runs everything named test_*.py and *_test.py so the test source files can be simple and isolated from each other. There can be multiple directories in test/fplan for specific tests and their data. The example above contains some simple tests for the Data class load_file() function. Adding some solver tests seems like an obvious next step. Repeat as necessary until we have good coverage.