Add calculate_cng_indices

[sjawhar]

I implemented this function while working on a paper and decided it belongs in this excellent project. I will add tests (I have been comparing to output from R), but there is a major potential issue in the code which I will mark out with inline comments.

[jbiggsets]

Thanks so much for submitting this PR! I had a few comments/questions.

Let me know if any of them don't make sense, or if you just want me to submit a PR to merge into this branch.