A very fast static spatial index for 2D points based on a flat KD-tree,
using https://github.com/mourner/kdbush.hpp
Compared to QgsSpatialIndex, this index:
- supports single point features only (no multipoints)
- is static (features cannot be added or removed from the index after construction)
- is much faster!
- supports true "distance based" searches, i.e. return all points within a radius
from a search point
