DEP: deprecate stats.oneway.  Closes gh-2569.

`oneway` is broken, badly documented and a duplicate of `f_oneway`.
DEP: deprecate stats.glm and stats.cmedian.

Closes gh-580 (cmedian Statistics Review ticket).

[ci skip]  (just rebasing, Travis green already)
Merge pull request #2733 from rgommers/deprecate-oneway

DEP: deprecate stats.oneway.  Closes gh-2569.