Refactoring, small improvements, and cleaning notation of IsotonicRegression
Changing docs from "vector" to "1d array"
DOC: fix 2 covariance examples rst math markup.

Some formulas in plot_mahalanobis_distances and
plot_robust_vs_empirical_covariance were not correctly rendered by sphinx
as some characters were not properly escaped. This is fixed by setting the
docstrings as raw string using r"""

There was also missing braces for the number of features and samples subscript.
Branching 0.15.X to prepare the 0.15.0b1 release
TST test interaction features outside doctest
Minor fixes in the docs
Fix some fun column span alignment errors.
Reverting array X to vector x rename
DOC fix deprecation in CD
DOC missing verb in README
Merge pull request #3260 from larsmans/cd-deprecation

[MRG] fix faulty deprecation in CD
DOC : clarify error message in _binary_roc_auc_score
TST silence spectral test/test warning message
FIX wrong parameter name in deprecation warning
Docstring addition and x->X
TST reduce max_depth for numerical stability
Merge pull request #3261 from larsmans/test-interaction-features

TST test interaction features outside doctest
removed unused variables and imports, add used imports where missing.
COSMIT TYPO
Stop using some deprecated interfaces.