ENH: linalg: Add the `axis` keyword to linalg.norm.

Also fixed a bug that occurred with integer arrays and negative ord.  For example,
norm([1, 3], -1) returned 1.0, but the correct value is 0.75.
MAINT: linalg: removed unused imports from linalg.py
DOC: linalg: Improve the docstring of the new norm function.
BUG: fix typo on npy_config.h include guard

Was overlooked as loops.c.src where np.isnan lives includes it directly.
Now all isnan calls in numpy profit from the improvement.
Merge pull request #3394 from juliantaylor/incl-guard-fix

BUG: fix typo on npy_config.h include guard
DOC: add to HOWTO_RELEASE to check that the benchmark suite runs.
ENH: linalg: allow the 'axis' argument of linalg.norm to be a 2-tuple, in which case matrix norms of the collection of 2-D matrices are computed.
STY: linalg: some PEP8 clean up.
TST: linalg: loosen the requirements of some tests in test_blasdot.py.  Addresses part of gh-3399.
DOC: remove module index from reference guide.

It was broken, and after fixing it looked quite useless.
Update in conf.py is because `html_use_modindex` is deprecated.
DOC: add missing release notes to the reference guide.
Merge pull request #3401 from rgommers/remove-module-index

Remove module index from docs, add missing release notes.
Merge pull request #3400 from WarrenWeckesser/test-blasdot-failure

TST: linalg: loosen the requirements of some tests in test_blasdot.py.  ...
MAINT: linalg: simplify the computation of the svd-based matrix norms, and tweak the norm function's docstring.
Merge pull request #3387 from WarrenWeckesser/norm-axis

ENH: linalg: Add an `axis` argument to linalg.norm
typo
Merge pull request #3402 from argriffing/patch-3

typo
Merge remote-tracking branch 'upstream/master'
Fix some style errors as per seberg Jun6