functions for core statistics package

[Carnë Draug]

After the discussion at OctConf, I've looked at the forge statistics
package to see which ones would be included in the core package and
which ones we have to make a decision about.

I have made four lists in the wiki [1] for functions that should be
ready to go, and functions that need a decision.

Carnë

[1] https://wiki.octave.org/Statistics_package#Core_package