Minor refactoring of VoroinoiGrid #2437
Description
VoroinoiGrid contains a cell_coloring_property. This is visualization specific and does not belong in the grid class, but should be part of some kind of grid drawing/property layer portrayal of voroinoi grids.
Moreover, visual inspection in #2340 suggests that something might be at least visually off about voroinoi grids becasue agents don't appear cleanly in the center of the cell.