Isotonic regression (#170)

* Update mix.installs

* Add isotonic regression

* Add suggestions from code review

* Apply suggestions from code review

Co-authored-by: José Valim <[email protected]>

* Format

* Remove warning

* Add support for decreasing sequences

* Format

* Add Isotonic regression to docs

* Add :auto option to Isotonic Regression

* Use :increasing? as a tensor

* Suppress warning

* Update lib/scholar/linear/isotonic_regression.ex

Co-authored-by: José Valim <[email protected]>

* change :increasing? to increasing

* Pop unused value from opts

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Co-authored-by: José Valim <[email protected]>