FIX: Set correct impurity values in BestFirstBuilder
PEP8
Merge pull request #3049 from glouppe/tree-bestfirst

[MRG+1] Trees: set correct impurity values in BestFirstBuilder
target is response, not explanatory
typo: "fot" -> "for"
Merge pull request #3055 from ajschumacher/patch-3

typo: "fot" -> "for"
Merge pull request #3053 from ajschumacher/patch-1

target is response, not explanatory