Update plot_tree_regression_multioutput.py

Import statements have been moved to the top of the file.
FIX remove reference cycles from Tree
FIX dtype refcount; block resizing after building
Merge pull request #2789 from ugurthemaster/patch-2

Update plot_tree_regression_multioutput.py
svm fit numpy array indexing deprecation warning fix.
Merge pull request #2791 from adrinjalali/master

svm fit numpy array indexing deprecation warning fix.
Remove 'locked', add comments
Merge pull request #2790 from jnothman/tree_without_cycles

[MRG] Avoid reference cycles in Tree
DOC + FIX mean_ in PCA

Wasn't documented, and RandomizedPCA set it in __init__ instead of fit.
ENH faster heapsort in trees

Uses the heapsort version that I'm familiar with: linear-time heapify
followed by n delete-min operations. Also cache-friendlier by copying
active features into a temporary array.

Random forest training time on covertype:
now     34.4123s
before  60.1179s
ENH introsort in tree learner

Covertype w/ random forest training time down to 29.0857s.
ENH Make PresortBestSplitter cache friendly + cosmetics
fix numpy deprecationWarning('using a non-integer number ...') in svm
Merge pull request #2794 from cli248/master

fix numpy deprecationWarning('using a non-integer number ...') in svm
Proper centering of alpha_grid for sparse matrices
FIX: Normalize=True
update what's new
Merge pull request #2747 from larsmans/tree-sort

[MRG] faster sorting in trees; random forests almost 2× as fast
clarify what's new