DEP: put back veccdf/vecfunc/vec_generic_momemt in stats.distributions.

Removed erroneously in gh-3243 for 0.14.0.  These methods were always
private ones but not marked as such, and as the comments on gh-3243 show
they were used. Hence now deprecated.
Merge pull request #3685 from rgommers/stats-veccdf

DEP: put back veccdf/vecfunc/vec_generic_momemt in stats.distributions.