used by Cartesian_converter/Homogeneous_converter, where we
absolutely must not throw.
TODO : find a way to keep the assertion while having the
converters still work. (should types come with their
own conversions, like they all have .bbox() ?)
The additional dummy "int" specifies that the 2 points are
the lower-left and upper-right corner. This is more efficient
when one knows they are already in this configuration.
Same thing for Iso_cuboid_3, and the functors.
Use them in Cartesian_converter and Homogeneous_converter.