ENH: improve stats.nanmedian more by assuming nans are rare

Move nans to end of array instead of creating a new array without them.
This is faster under the reasonable assumption that nans are rare.
ENH: add runtime warning for all nan slice of nanmedian
MAINT: tweak cython variables declarations to avoid the warning
'Non-trivial type declarators in shared declaration ...' that is
generated by cython 0.20.
MAINT: special: tweak the generated cython variable declarations in _ufuncs.pyx
to avoid the warning 'Non-trivial type declarators in shared declaration ...'
that is generated by cython 0.20.
GEN: special: regenerate _ufuncs.pyx
Merge pull request #3430 from WarrenWeckesser/cython-declarations

MAINT: Fix some cython variable declarations to avoid warnings with cython 0.20.
MAINT: ignore harmless whitespace errors in pep8
MAINT: enable pep8 in travis-ci
WHT: fix remaining pep8 whitespaces
Merge pull request #3396 from juliantaylor/nanmedian-improve2

ENH: improve stats.nanmedian more by assuming nans are rare
Merge pull request #3273 from pv/pep8

Add PEP8 check to Travis-CI
BUG: modified ClusterNode.pre_order() for non-root nodes.

Fixes gh-2177 by replacing the boolean vectors with sets.
TST: add test for subtree pre_order in cluster.hierarchy.

Tests that ``pre_order`` method works for subtree of ClusterNode root.
MAINT: minor PEP8, test docstring cleanup for previous test_hierarchy commit.
Merge branch 'pr/3394' into master.  Closes gh-2177.

See https://github.com/scipy/scipy/pull/3394