Add multi-class confusion matrix metrics

[feifjiang]

Added confusion matrix calculation for multi-class classification, including:

1. confusion matrix for the top n most frequent labels
2. misclassification per class

[salesforce-cla]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): FeiFei Jiang <f***@f***.i***.s***.com>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Salesforce.com Contributor License Agreement and this Pull Request will be revalidated.

[codecov]

Codecov Report

Merging #532 (a99fa5a) into master (13ad9cd) will increase coverage by 86.77%.
The diff coverage is 95.23%.

@@             Coverage Diff             @@
##           master     #532       +/-   ##
===========================================
+ Coverage        0   86.77%   +86.77%     
===========================================
  Files           0      347      +347     
  Lines           0    12018    +12018     
  Branches        0      399      +399     
===========================================
+ Hits            0    10429    +10429     
- Misses          0     1589     +1589     

Impacted Files	Coverage Δ
...op/evaluators/OpMultiClassificationEvaluator.scala	95.59% <95.23%> (ø)
...rce/op/stages/impl/preparators/SanityChecker.scala	90.57% <0.00%> (ø)
...scala/com/salesforce/op/testkit/RandomVector.scala	90.47% <0.00%> (ø)
...op/stages/impl/feature/TimePeriodTransformer.scala	100.00% <0.00%> (ø)
...m/salesforce/op/aggregators/ExtendedMultiset.scala	75.00% <0.00%> (ø)
...in/scala/com/salesforce/op/testkit/RandomMap.scala	100.00% <0.00%> (ø)
...a/com/salesforce/op/cli/gen/ProjectGenerator.scala	87.50% <0.00%> (ø)
...n/scala/com/salesforce/op/testkit/RandomText.scala	100.00% <0.00%> (ø)
...a/com/salesforce/op/testkit/PartiallyDefined.scala	100.00% <0.00%> (ø)
...a/com/salesforce/op/test/FeatureTypeEquality.scala	80.00% <0.00%> (ø)
... and 338 more

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[tovbinm]

@gerashegalov are you having fun?! ;)

[gerashegalov]

@tovbinm yes. some fun to trick the CLA bot with @feifjiang