WIP draft of the Statistics concept

[colinleach]

I have no idea how we would create an exercise to go with this. Too divisive: half the students (of the subset interested in Julia) already know this stuff and get bored with things that seem self-evident; the other half would need a semester-long course to understand it.

I'm not sure it matters much. If this gets merged, we can link to it from other Concepts, as I already did from randomness and will probably do from one I've not written yet (but may be called vector-operations).