ENH less copying in validation for neighbors
FIX safe_asarray to handle LIL, DOK formats
TST safe_asarray for dok_matrix and lil_matrix
DOC: website: feature selection on front page

Replaces Isomap. Fixes #3033.
FIX loss function example

The Python API to the loss functions was lost in the last refactor.
Restored the example by defining the losses inline.
Fixed the x-axis to show "f(x)" instead of "y * f(x)".
Removed references to private APIs.
DOC: clustering: merge discussions of k-means and inertia

K-means is hard to understand without knowing what inertia is,
and we still don't have a public function for computing it.
Gets rid of an ugly TODO in the docs.
MAINT remove unsupported documentation formats

In particular, "htmlhelp" and "qthelp". None of the core devs uses
this or is willing to support it. Fixes #2512 by not fixing it.
DOC: move mean_shift docs to MeanShift

The class is linked from the clustering docs, while the function
isn't. Also standardized the spelling on "mean shift". StudlyCaps
version is an artifact of PEP8 class naming.
Merge pull request #2952 from jnothman/future_warn_two_labels

MAINT warn of future behaviour change proposed in #2610
ENH factor out squared norm helper
Now capable of treating sample_weights in feature space
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
FIX: weight impurity improvement by the size of the node
DOC: better doc for impurity_improvement