Feature/ttest criterion

[volico]

Proposed changes

Hi!

I added T-test splitting criteria from Su, Xiaogang, et al. "Subgroup analysis via recursive partitioning." Journal of Machine Learning Research 10.2 (2009). as discussed in #530

In #562 this criteria is added only for binary trees and two-class problems, my PR contributes same splitting criteria for regression task.

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• Bugfix (non-breaking change which fixes an issue)
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Checklist

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Further comments

[jeongyoonlee]

LGTM. Thanks for the contribution!