Revert mass exodus of statistics functions

[Carnë Draug]

For the upcoming Octave version, we moved a bunch of Octave functions
from core to the statistics Forge package.  We are also now planning
on having a statistics package as part of core.

This is already causing trouble because we don't a have a release of
the forge package to cover the removed functions.  What if we revert
that change and instead move the functions to the core package for 4.6
release?  This would be less disruption to the end user and avoid the
complications on the current forge package.

Carnë