add hexagonal binning

[HDembinski]

add a 2d axis which does hexagonal binning

Everything about hexagonal grids: https://www.redblobgames.com/grids/hexagons/

[ryanelandt]

After axis growing, the values in the old bins need to be assigned into the new bins. I think the way this is currently done here uses an integer shift for each axis to calculate the new index for each bin. This approach will require modification/generalization to support rebinning a 2D axis. Do you envision a 2D hexagonal axis type supporting axis growing?