add array function auc

[taiyang-li]

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Changelog category (leave one):

• New Feature

Changelog entry (up to few sentences, required except for Non-significant/Documentation categories):

• Add array function auc(used in machine learning). See more details:
  https://developers.google.com/machine-learning/crash-course/classification/roc-and-auc

[taiyang-li]

The Two failed integration tests seem not caused by my commit.

[alexey-milovidov]

@taiyang-li I did not understand the code in the inner loop.
I tried to imagine the algorithm and have rewrite it to something more simple:
27f0b93

I'm not sure that it's 100% correct (I'm not into machine learning) but your tests have passed.