Second patch to implement the zoomlevel function.
This patch will modify the saveGeometryNode so that if zoomlevels are enabled in the configuration, it will store a ZOOM_LEVEL integer in the spatial index database
if the ZOOM_LEVEL column does not exist yet, an exception is generated and the column is added with an ALTER TABLE sql query
then we try again with the query
The integer stored is 0 if the Node fit in all the BBox of the zoomlevels.
It is one if it doesn't fit in the smaller one but fit in all the others