FIX: compute weighted oob scores

…scikit-learn#7838)

* initial commit for return_std

* initial commit for return_std

* adding tests, examples, ARD predict_std

* adding tests, examples, ARD predict_std

* a smidge more documentation

* a smidge more documentation

* Missed a few PEP8 issues

* Changing predict_std to return_std #1

* Changing predict_std to return_std #2

* Changing predict_std to return_std #3

* Changing predict_std to return_std final

* adding better plots via polynomial regression

* trying to fix flake error

* fix to ARD plotting issue

* fixing some flakes

* Two blank lines part 1

* Two blank lines part 2

* More newlines!

* Even more newlines

* adding info to the doc string for the two plot files

* Rephrasing "polynomial" for Bayesian Ridge Regression

* Updating "polynomia" for ARD

* Adding more formal references

* Another asked-for improvement to doc string.

* Fixing flake8 errors

* Cleaning up the tests a smidge.

* A few more flakes

* requested fixes from Andy

* Mini bug fix

* Final pep8 fix

* pep8 fix round 2

* Fix beta_ to alpha_ in the comments

* add test for _preprocess_data and make it consistent

* fix pep8

* add doc, cast systematically y in X.dtype and update test_coordinate_descent.py

* test if input values don't change with copy=True

* test if input values don't change with copy=True #2

* fix doc

* fix doc #2

* fix doc #3