add cmh and breslow-day tests

[annepym]

Added a separate file including functions for Cochran-Mantel-Haenszel and Breslow-Day tests, per Kelly's request. @nickdeveau94 this is mostly your code.

[VishalMurali]

@annepym Why are these tests in a different file? We have a file called stats.py for the various statistical tests. It would be convenient to have all our statistical tests in one place.

[dwww2012]

i'm wondering generally if an entirely new file is required. regardless, some small things like variables names and documentation should be improved before merging

[VishalMurali]

Can we add an example of these tests in one of our example notebooks? The purpose is to clarify how this test would used in a typical machine learning problem.

[deelin]

Is this going to be merged in soon? It'd be awesome to actually use this in the upcoming client analysis. (I think this is a public repo, so won't say who the AI analysis is for :) )

[ljbaker]

@annepym , totally agree with @deelin. Can you commit the suggestions? We can add examples of the CMH test in a separate commit.

[ljbaker]

@deelin , @annepym added some intense docstrings and tests, clarified variable names when possible

[ljbaker]

@VishalMurali makes a good point that this should be moved to stats.py

[ljbaker]

I've added a notebook that walks through the functions with examples (CMH_test_example.ipynb). I'm going to defend that the utility functions for CMH should have a standalone folder within utils/cmh.py, as the utility functions could be confused with other functions (e.g. utils.crosstabs.crosstab_odds_ratio for calculating odds ratios in most other cases).

[dwww2012]

ok this looks good to me

[deelin]

WOOHOO!