BUG: io.netcdf: do not close mmap if there are references left to it

Fixes gh-3630
DOC: io: update netcdf docs vs mmap
DOC: io/netcdf: add example on mmap features, adjust doc

[ci skip]
TST: io/netcdf: remove invalid test

The "Too many open files" warning is actually unavoidable, since the
arrays refer directly to data in memory mapped files, and those files
cannot be closed as long as the arrays are around.
TST: io/netcdf: fix issues with warnings in tests

Ignore warning emitted in test_itemset_no_segfault_on_readonly.  Remove
a test using assert_warns; it doesn't clear warning registry, so the
warning in the other test makes this one not fire.
ENH: convert the cryptically named scipy.special information theory functions into ufuncs
TST: add negative values to scipy.special information theory function tests
DOC: improve scipy.special information theory ufunc docstrings
MAINT: add .pxd files
MAINT: simplify kl_div
MAINT: combine multiple ufuncs into the same .pxd file
[ENH] Add a step size parameter for optimize.check_grad
[TST] Add a unit test for optimize.check_grad
Merge branch 'check_grad', pull request #3945
ENH: add huber loss function to the collection of convexity-related ufuncs
PEP8: pacify the pep8 overseer
ENH: add pseudo-huber loss function ufunc
DOC: stats: percentileofscore references np.percentile
DOC: linalg: move the 'versionadded' info of 'pascal' to the notes.
BUG: linalg: pascal(35) was incorrect: last element overflowed np.uint64 data type.