Deprecate nanmean, nanstd, nanmedian in favour of the numpy equivalents

See #1727
TST: stats: filter out DeprecationWarnings in TestNanFunc
Latest changes from standalone idlsave:

- Properly handle null data
- Added support for description records
BUG: fix ndimage functions for large data

map_coordinates uses int to index into data arrays which can overflow
for arrays of several gigabytes size. Patch by @cgohlke.
some other occurances of int replaced by npy_intp for safety.
Closes gh-3733
Fix documentation error for nc chi-squared dist

Fix documentation error for the PDF of the non-central chi-squared continuous
distribution, which was described wrong but implemented correctly. Also do some
minor style fixes (remove spaces at line end).
Merge remote-tracking branch 'upstream/master' into chi2-documentation-fix
DOC: sparse: clarify CSC and CSR constructor usage

Same treatment as given to COO in 3f19f49b83223e826d2e7d97c7060773a8522a0b.

[ci skip]
MAINT: enable np.matrix inputs to solve_discrete_lyapunov
Merge pull request #4175 from larsmans/csc-csr-constructor-docstring

DOC: sparse: clarify CSC and CSR constructor usage
BLD: fix Bento build.  A removed file was referenced, see commit 542adf8b00.
TST: fix an intermittently failing test case for special.legendre

Test used assert_equal on floating point numbers.
Merge pull request #4145 from LifeIsHealthy/chi2-documentation-fix

DOC: Fix documentation error for nc chi-squared dist
Merge pull request #4117 from ev-br/pr/3925

deprecate nanmean, nanmedian and nanstd in favor of their numpy equivalents
Merge pull request #4177 from argriffing/solve-lyapunov-np-matrix

MAINT: enable np.matrix inputs to solve_discrete_lyapunov
Merge pull request #4179 from rgommers/fix-legendre-failure

TST: fix an intermittently failing test case for special.legendre
Merge pull request #4126 from astrofrog/latest-idl-fixes

scipy.io.idl: support description records and fix bug with null pointers
BUG: fix issue with logpmf method of stats distributions.
Merge branch 'pr/4049' into master.

See https://github.com/scipy/scipy/pull/4049
Merge pull request #4143 from juliantaylor/ndimage-large-data

BUG: fix ndimage functions for large data
ENH: release GIL in more ndimage functions