This function computes the bounding box for each polygon and adds this information
to the list. The bounding boxes can be used in various applications.
Our main motivation is for the massive PointsInPolygon search to exclude those
polygons as candidates whose bounding box does not contain the current point.
Usage
AddRanges(poly.list)
Arguments
poly.list
polygon list with three elements: data, polys, and poly.centers
Value
Returns augmented polygon list with additional element -- "ranges"