DOC: fix several Sphinx warnings when building the docs

patch developed with Dmitrijs Milajevs and Ioannis Tziakos
DOC: change example sample spacing

Change the sample spacing in the example so the linear interpolation points line up with the original points
BUG: linalg: Remove a couple shadowed (and buggy) tests from test_decomp.py.
MAINT: linalg: Remove some unused variables from test_decomp.py.
MAINT: sparse: Clean up in bench_sparse.py:

* Remove unused imports.
* Don't use 'import *' form of import.
MAINT: sparse: Remove unused imports and unused variables.
MAINT: sparse: Remove some more unused imports and unused variables.
TST: stop shadowing ellip* tests vs boost data
MAINT: special: use NPY_PI, not M_PI

M_PI is commonly available, but non-standard, so certain compiler flags may
hide it.
BUG: io.arff: Fix ARFF reader crashing when a comment line follows a blank line. Closes gh-2088
Merge pull request #4258 from mfeurer/master

BUG: io.arff: Fix ARFF reader crashing when a comment line follows a blank line.
MAINT: signal: PEP8 and related style clean up.
Merge pull request #4250 from WarrenWeckesser/pyflakes-cleanup

MAINT: Some pyflakes-driven cleanup in linalg and sparse
Merge pull request #4261 from WarrenWeckesser/pep8-signal

MAINT: signal: PEP8 and related style clean up.
Merge pull request #4219 from ev-br/sphinx_warns

DOC: silence several Sphinx warnings when building the docs
Merge pull request #4254 from larsmans/m_pi

MAINT: special: use NPY_PI, not M_PI
TST: silence warning from testing optimize.curve_fit with infs.
TST: clean up optimize tests (no test docstrings, Github issue numbers).
TST: silence test warnings for invalids for stats.nanmean/nanstd.
Merge pull request #4263 from rgommers/clean-testnoise

MAINT: clean up test noise and optimize tests for docstrings and Trac issue numbers