GitLab

* Add R-squared metric, documentation, and tests

* Fix static ci checks: correct Sklearn r2_score URL link, fix cpp linting errors

* Replace non-static data members from pragma reduction with variables

* Update src/metric/metric.cpp

* Add additional asserts, remove needless y.copy()

* Add back y.copy()

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Co-authored-by: James Lamb <jaylamb20@gmail.com>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>