MAINT: clean up comments/commented-out code in distributions.py; some PEP8 fixes.
MAINT: enable rdist._cdf, making it a lot faster.  Closes gh-1285.

Issue for hyp2f1 that is worked around is discussed in gh-1561.
ENH: add central moment methods to invweibull and loglaplace distributions.

Faster, more accurate and removes a warning generated by the distribution fit
test for invweibull.
TST: speed up stats tests some more, and add run_module_suite to each test file
Merge pull request #2612 from rgommers/stats-distributions-cleanup

MAINT: clean up comments/commented-out code in distributions.py
BUG: stats: Handle mu=0 correctly in poisson.pmf. Closes gh-2618.
TST: stats: add a test of broadcasting for stts.poisson.pmf
MAINT: special: convert tabs to spaces in cephes/pdtr.c
ENH: special: update the cephes functions pdtr and pdtrc to handle m=0.
ENH: special/cdflib: Allow XLAM==0 when WHICH==2 in cdfpoi()
TST: special/cdflib: Add a test of pdtrik with mu==0
BUG: stats: fixed _argcheck in poisson to handle an array argument, and to set self.b to 0 when mu==0.
BUG: stats: The support bounds a and b might be arrays, so flatten them when used as the third argument of np.place