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.
DOC: stats: percentileofscore references np.percentile
MAINT: use isMaskedArray instead of is_masked to check type
Merge pull request #3998 from argriffing/is-masked-usage

MAINT: use isMaskedArray instead of is_masked to check type
Merge pull request #3993 from clarkfitzg/stats_docs

DOC: stats: percentileofscore references np.percentile
Merge pull request #3960 from pv/netcdf-fix

BUG: io.netcdf: do not close mmap if there are references left to it
ENH: min(max)imum_filter1d using the MINLIST algorithm, fixes #3898

This PR implements the minimum_filter1d and maximum_filter1d functions
using Richard Harter's implementation of the MINLIST algorithm. While
slower (~30%) than the MINLINE algorithm currently in place for random
inputs, its performance is consistent for all inputs, and is much
faster for long, ordered seqeunces.
Merge branch 'pr/3527' into master.

Review at https://github.com/scipy/scipy/pull/3527
MAINT: optimize: remove a couple unused variables in zeros.c
BUG: Fix C90 compiler warnings in `NI_MinOrMaxFilter1D`

Variable declarations were preceded by code due to a rebase.
Merge pull request #4006 from jaimefrio/c90_warnings

BUG: Fix C90 compiler warnings in `NI_MinOrMaxFilter1D`
MAINT/DOC: Fix spelling of 'decomposition' in several files.
Merge pull request #4007 from WarrenWeckesser/doc-spelling

MAINT/DOC: Fix spelling of 'decomposition' in several files.
Merge pull request #4004 from WarrenWeckesser/maint-zeros-c

MAINT: optimize: remove a couple unused variables in zeros.c