Shows tooltips with statistics in the column headers

@jpsim @astigsen 

int, float and double: min, avg, max, sum (about 20ms)
date: min, max (about 10ms)

https://app.asana.com/0/15028213105325/15300662627859

I did try padding the strings so they could be right aligned, but the commonly proposed method for changing the font of tooltips (to a monospace one) doesn't seem to work.
This is only moved up to public methods, it used to be a private property but in order to eliminate the helper methods, we needed a way to access the underlying array from outside.