* Refactor the index scale selection.
* Document more explicitly where coordinates are encoded or not.
* Improve error messages part of the encoding / decoding and of space
projections.
* Push down the parameters in order for filtering to be done as early
as possible w.r.t the view port.
* Add view_port-based filtering in the lower-layers of the DB.
* Implement PartialOrd on Position, to simplify point in box checks.
* Implement multi-scale indices generation.
* Deduplicate values before sorting them while generating an index, to
reduce the number of points to sort.
* Use a hastable to deduplicate values, instead of a sort + dedup call.
* Minor code cleanups
