Test coverage

the test coverage from covr is ~71% but it seems like the coverage is somewhat more accurately ~4%. tests/testthat/test_CVtreeMLE_breastcancer.R runs an example but doesn't include any assertions and doesn't inspect the output of the run (and in fact emits a warning on my system. due to how the test files are structured, the only function that seems to have substantial coverage right now is simulate_mixture_cube.

with the above, I find it very challenging to evaluate the functionality of the software. please expand the test coverage as necessary to ensure the majority of the functions are running not just end to end, but as expected when run.

[blind-contours]

I have included examples in the documentation that run an example of each major function. I've added and am still adding more testing. With the breast cancer test, our goal is to ensure that the output interactions are in the correct format - I've updated this test script accordingly. Other tests have been included to ensure we are getting, for a respective variable set, the rules that have the maximum explanatory on the outcome in the iterative backfitting procedure. I'm adding some additional tests for the estimators as well.