hexbin objects. Basic components are
a cell id and a count of points falling in each occupied cell.
Basic methods are show(), plot() and summary(), but also erode.hexList(x, y = NULL, given = NULL, xbins = 30, shape = 1,
xbnds = NULL, ybnds = NULL, xlab = NULL, ylab = NULL)xycoerce method to produce
hexbinList objects from lists.hexbin, hdiffplot