Bug fix in computing the dataset_centroid in NearestCentroid

A bit more of vectorization and minor code optimizations were added
ENH: Added median_absolute_error to metrics
DOC: Added doc in model_evaluation.rst
ENH: unit tests for median_absolute_error
ENH: test_common.py unit tests pass
FIX: Small typo
Merge branch 'master' into median_absolute_error

Conflicts:
	doc/modules/model_evaluation.rst
TST make catch_warnings blocks more robust
Merge pull request #3761 from FlorianWilhelm/median_absolute_error

add median absolute error (MAE) regression metric
Merge pull request #3750 from MechCoder/bug_nearest_centroid

[MRG+2] : Bug fix in computing the dataset_centroid in NearestCentroid
ENH added digits as optional arg for classification_report
FIX warning check in test_affinities
Merge pull request #3763 from ogrisel/fix-warning-test-affinities

[MRG+1] FIX warning check in test_affinities