Merge pull request #3157 from mjbommar/isotonic-increasing-auto

[MRG] Determine IsotonicRegression ``increasing`` by Pearson or Spearman corr rho
Additional docstring fixes for issue #3167
Merge pull request #3197 from mjbommar/issue-3167-eradicate-todense

Additional fixes for .todense() / issue #3167
Merge branch 'master' of https://github.com/scikit-learn/scikit-learn