Proposal: Add OR operator to predicates (Rtree)

Hello,

Using Rtree, I fell into the case where I need to know whether a box is within another box OR overlaps it. The current implementation doesn't allow this operation.
In my understanding, I can't see any reason not to allow it, as AND and NOT are allowed.
We can't even use De Morgan's law (`A || B = !(!A && !B)`) as we can't negate a whole expression (AND operator returns a boost::tuples::cons and not another predicate with the NOT operator implemented).

Maybe there is a limitation I can't see, if there is, I'll be happy to know it ;)
Thanks !