Add Thiel's U Statistic (Uncertainty) Metric

[shenoynikhil]

What does this PR do?

Implements Theil's U statistic for nominal (categorical) data.
Fixes #1273

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[SkafteNicki]

LGTM

[SkafteNicki]

@stancld please re-review :]

[codecov]

Codecov Report

Merging #1337 (51e21d3) into master (1ab660e) will decrease coverage by 53%.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1337     +/-   ##
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- Coverage      87%     33%    -53%     
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  Files         208     210      +2     
  Lines       11695   11765     +70     
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- Hits        10134    3921   -6213     
- Misses       1561    7844   +6283     

[stancld]

LGTM! Very nice, thanks for the contribution!

[stancld]

Also, can we add a link for this metric to docs/source/links.rst? -- https://en.wikipedia.org/wiki/Uncertainty_coefficient

[shenoynikhil]

@stancld @SkafteNicki I was wondering why in each of these (crammers_v, pearson, theils_u), we have the exact same update function. Would it make sense to have a common one so that it can be put into utils.py. I am referring to the following functions,

• _pearsons_contingency_coefficient_update
• _cramers_v_update
• _theils_u_update