Returns the height of a 3D geometry at a particular point.
If the point does not coincide with a vertex on the 3D geometry, then the nearest vertex is chosen.
Parameter 1 - a 3D geometry.
Parameter 2 (Optional) - a 2D geometry specifying the point on the 3D geometry whose height is to be returned. If this parameter is omitted, then a vertex on the 3D geometry is chosen at random.
Returns: the height of the nearest vertex on the 3D geometry.
If parameter 1 is not a geometry, an exception will be thrown.