predicates::satisfies doesn't(?) allow control over tree traversal

AFAICT, `predicates::satisfies` lets you give an arbitrary function object and then use it to, say, visit every `rtree::value_type` in an `rtree`. It seems like it just needs a second function, which could default to `[](auto&&...) { return true; }`, to let you control the traversal of the tree. For example, suppose I want to visit an `rtree` and visit all points in the (-,+) direction of the line y=x. I think(?) there's no way to do that right now. If I have `struct IsAboveLeft { bool operator()(const Point2D& pt) const { return pt[0] - pt[1] > 0; } };` then I could use `predicates::satisfies`, but it would be an O(n) query of my `rtree`. But if I could do something like
```
struct FindAboveLeft {
bool partially_contains(const Box&) const; // check if ay part of AABB is above-left of the line y=x
bool contains(const Value&) const; // check if the point is above-left of the line y=x
};
// ...
rtree.query(satisfies(), std::back_inserter(results));
```
then the query could be O(log n) per result returned. As it is, I can't see how to get this behavior without injecting things into the `boost::geometry::index::predicates::detail` namespace. I think a predicate like this could be very expressive and allow for all sorts of interesting queries.