FIX: ward_tree now returns children in the same order for both
structured and unstructured versions

Add test for this in test_hierarchical
Add forgotten newline
Cosmetic change in test
ENH: Add metric support to neighbors_graph
Remove optional input validation from pairwise_metrics
FIX bug in fit_params assumed to be an array

Fixes gh-3957.
DOC: make LinearSVR appear in the doc reference
DOC fix/expand Imputer example docs

Fixed formatting of Script output in text introduction.

Added more details to the introduction on use cases for imputing.

Fixes gh-3918.
Minor changes: float conversion and rename rnd into rng
Validation of params passed to neighbors_graphs
Merge remote-tracking branch 'upstream/master' into wtree_children

* upstream/master:
  DOC fix/expand Imputer example docs
  DOC: make LinearSVR appear in the doc reference
  FIX bug in fit_params assumed to be an array
  Replace feature with sample
  FIX: Raise AttributeError for fit_predict
  Fixed a bug in RFECV when step != 1
  facecolor/edgecolor are passed arguments to savefig if they differ from rcParam['figure.facecolor'] From nilearn
  MRG: Fix FeatureAgglomeration docs

Conflicts:
	doc/whats_new.rst
Merge pull request #3969 from MechCoder/remove_check_X_y

[MRG+1] Remove optional input validation from pairwise_metrics
Merge pull request #3968 from MechCoder/metric_neighbors_Graph

[MRG+2] ENH: Add metric support to neighbors_graph
Merge pull request #3966 from mvdoc/wtree_children

FIX: ward_tree returns children in the same order for both structured and unstructured versions