enable choice of cut method

[mwcz]

median-cut and mean-cut are both built-in, but it's not possible to choose which one. add a way to choose whether to use mean or median for choosing where to cut each box.

other ways to choose the cut location could be added. mode-cut would be easy but probably useless. cutting based on the 1st, 2nd, or 3rd stdev could be very useful.