[stats] Allow for keyword arguments in distributions.

Generates the correct signatures for args, loc, scale
dynamically for each distribution. Thus sidesteps the
using _fix_loc_scale_kwarg3 at each call.
Generates signatures for stats and moments as well,
allowing `size` and `moments` as keywords or
positional arguments.
TST: Change the test allowing propagation of nans with
incorrect number of parameters. The changed behavior matches
that of R.
TST: test all distributions
TST: refactor tests, remove unreacheable code
MAINT: some minor edits to PR-2588, style and update gh-issue number.
MAINT: collected the inspect calls together
MAINT: bypass inspect in stats.distributions by using explicit shapes
DOC: removed inconsistencies between shape parameters in docstrings.
MAINT: small refactoring. pep8 compliance, more robust tests
DOC: document the changes to the shapes parameter.
TST: add tests for subclassing stats.rv_continuous.
TST: tests for check for *args & **kwds w/inspected shapes
BUG: Change working dir when building extension.

Current working directory should be changed (to trusted one) when building
an extension so compiler accidentally won't pick up anything from (untrusted)
current directory.
MAINT: interpnd.pyx is not autogenerated
BUG: qhull: fix potential infinite loop in RidgeIter2D

For certain triangulations, RidgeIter2D could enter an infinite loop due
to degenerate simplices.  This commit replaces the RidgeIter2D by an
explicit list of neighboring vertices.

An explicit list is also more useful for higher-dimensional data.

Fixes gh-2359
BUG: odr: avoid memcpy to the same address

Silences a Valgrind warning.
MAINT: attach _parse_args to the instance, not class.

Fixes the test failure in statsmodels induced by this PR/2588.
Attaching to the instance a kind of makes sense since
shapes/numargs are only created in __init__ anyway.
TST: rework tests for explicit shapes
Merge pull request #2660 from TomasTomecek/weave-build-cwd

BUG: Change working dir when building extension.
Merge pull request #2699 from pv/fix-odrpack

BUG: odr: avoid memcpy to the same address