Added exhaustive CHAID

[Rambatino]

Run with --exhaustive flag e.g:

python -m CHAID tests/data/titanic.csv survived sex embarked --max-depth 4 --min-parent-node-size 2 --alpha-merge 0.5 --dependent-variable-type continuous --exhaustive

This feature comes from a request from #112 - to make an exhaustive selection of the choices.

[Rambatino]

Fixes #112

[codecov]

Codecov Report

Merging #113 into master will decrease coverage by 0.22%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
- Coverage   93.13%   92.90%   -0.23%     
==========================================
  Files           8        8              
  Lines         626      634       +8     
==========================================
+ Hits          583      589       +6     
- Misses         43       45       +2     

Impacted Files	Coverage Δ
CHAID/stats.py	96.51% <90.00%> (-1.08%)	⬇️
CHAID/invalid_split_reason.py	100.00% <100.00%> (ø)
CHAID/tree.py	96.80% <100.00%> (+0.02%)	⬆️

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