This function creates an object representing
a multi-dimensional rectangular parallelepiped (box)
with sides parallel to the coordinate axes.
The object can be used to specify the domain of a multi-dimensional
point pattern (see ppx) and in various
geometrical calculations (see volume.boxx,
diameter.boxx, eroded.volumes).
The optional argument unitname specifies the name
of the unit of length. See unitname
for valid formats.