Returns a representation of space that encloses this Boundable, preferably
not much bigger than this Boundable's boundary yet fast to test for intersection
with the bounds of other Boundables. The class of object returned depends
on the subclass of AbstractSTRtree.
an Envelope (for STRtrees), an Interval (for SIRtrees), or other object
(for other subclasses of AbstractSTRtree)