Updated Docs with Implementing your Own Test

[tathey1]

In light of issue 115:

• I added a cell in example.ipynb with "Use a custom test with StatTest," which is involves using custom code for a test (though it mostly relies on a scipy.stats function)
• I added a README in the stats submodule, and included it in the sphinx docs with a general overview of how to do it.

[trevismd]

Thanks a lot for the PR, it looks great!
I left a few suggestions if you'd like to have a look at them, otherwise they are memo for me afterwards when this is merged. I see that you understand sphinx/rst structure better than I do. (If you'd like to help users by further improving the autogenerated documentation, I'm sure it would be much appreciated.)

For the notebook, I think we could highlight the difference with the previous cell with a small comment above the function definition, explaining that we could virtually use anything.
I have another example in another PR where I show how to support 1-sample tests, maybe I'll add this example too later on, just for demonstration.

Thanks again, and let me know if I can be of any help of course!

[tathey1]

@trevismd I agree with all your suggestions. I addressed them all, I think.

[codecov]

Codecov Report

Merging #117 (77d5033) into dev (2a5ebe1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #117   +/-   ##
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  Coverage   97.04%   97.04%           
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  Files          29       29           
  Lines        1898     1898           
=======================================
  Hits         1842     1842           
  Misses         56       56           

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