ENH: stats: Some enhancements to the chisquare functions:

* Add the `axis` argument to stats.chisquare and stats.mstats.chisquare.
* Add the `ddof` argument to stats.mstats.chisquare.

Closes gh-2108 (trac #1583).
Modified the interpretation of the axis argument.

It is now the axis of the array resulting from broadcasting f_obs
and f_exp.

Also simplified the code a bit.
Update the docstring for mstats.chisquare, and change the casting of ddof in mstats.chisquare so the p value data type is correct when axis=None is used.
MAINT: stats: Improve chisquare docstrings, and add tests that compare stats.mstats.chisquare to stats.chisquare.
Merge pull request #2480 from WarrenWeckesser/stats-chisquare

ENH: stats: Some enhancements to the chisquare functions: