improve performance of within with large radii

for large radii, the number of buckets that have to be looked at
when doing a HasWithin starts to dominate query performance. To
work around these cases we now do the following two things:

  -  store the bounding box for the buckets and intersect the cube
     of buckets which have to be traversed with the bounding box.
  -  when the number of buckets which have to be traversed gets
     larger than the total number of buckets in the organizer,
     loop over all buckets instead of looping over the cube.