   Crosses(g1,g2)
Returns 1 if g1 spatially crosses g2.
Returns NULL if g1 is a Polygon or a MultiPolygon,
or if g2 is a Point or a MultiPoint.
Otherwise, returns 0.

The term /spatially crosses/ denotes a spatial relation between two given
geometries that has the following properties:


 --- The two geometries intersect

 --- Their intersection results in a geometry that has
a dimension that is one less than the maximum dimension of the two given
geometries

 --- Their intersection is not equal to either of the two given geometries
