kd-tree updates and logarithmic method

[gaperez64]

I started implementing the new kd-tree building and querying algorithms. Two things are missing on the kd-tree front:

1. The trim procedure which will be used when implementing the logarithmic method data structure
2. Updated iteration through the elements of the tree, taking into account removed/dirty nodes. Both of the above are marked in the code as TODOs

[gaperez64]

One thought about item 2.

Perhaps the easiest is to keep a vector with the nonremoved elements and use it as a cache that is dropper after every trim operation that does remove at least one node?

[gaperez64]

The kd-tree structure is now fully updated. Moving on to implementing the logarithmic method next.

[gaperez64]

One thought about the logarithmic method: for unions we avoid rebuilding trees, but unbalance the ones already there. For intersections though... we still need to rebuild, so this is less promising. I will instead implement the dynamic version. And on that note, the action points are now:

• [x] remove the trim method and removed attribute of kd-tree nodes
• [x] implement a vekd-backed antichain which keeps track of the size of the set to decide whether to continue with lists or trees.