ENH sparsify and densify methods for CD models
FIX "ValueError: startprob must sum to 1.0" in HMMs

The reason for these error is that the value of self._covars_ is nan.
Adding a minimum denominator 1e-5 will solve these.
MAINT Rename over/underfitting example

Fixes #2834, #2845.
PY3 fixed examples
MAINT comment out unused import in example
MAINT full name for @h10r
FIX error message from trees for large inputs

Fixes #2809 (hopefully).
DOC: minor improvement in comments of an example
Revert "ENH sparsify and densify methods for CD models"

This reverts commit ee85da8421a35894620e86dbd5b0013b5628812f;
shouldn't have been committed to master.
Merge pull request #2864 from sciunto/doc

DOC: minor improvement in comments of an example
MAINT missing import_array() in isotonic r. Cython code

Not extremely important, but issues one less compiler warning.
MAINT get rid of compiler warnings from Liblinear
DOC no more "arithmetical order" for classes
BF: load_boston() return 13 features, but 14 feature names

Last column has the target value, hence only report first 13 names.
Fixes Issue 2751
Merge pull request #2865 from hanke/boston_13features

BF: load_boston() returns 13 features, but 14 feature names
ENH Cache features value for extra trees
FIX: Removed coef and improved initialization
Merge pull request #2866 from Manoj-Kumar-S/Iss2751

Fixes Issue 2751
FIX integer dtype for labels in DBSCAN

Fixes #2833.