An immutable RTree implementation

This is based on rectangles as spatial values from petomat/geom.

The following example shows how it works:

// based on petomat/geom
import de.petomat.geom._

// define our custom element type to be inserted into the rtree
case class Shape(name: String, r: Rectangle) { override def toString = name }

// some elements to work with
val A = Shape("A", Rectangle(Point(1, 1), Size(2, 3)))
val B = Shape("B", Rectangle(Point(4, 2), Size(3, 3)))
val C = Shape("C", Rectangle(Point(2, 2), Size(3, 6)))

// create an empty tree for elements of type shape
val t0 = RTree.default.Tree.empty[Shape]

// insert an element which creates a new tree
val t1 = t0.insert(A, boundingBox = A.r)

// to simplify insertion/deletion we place an implicit to extract the boundingBox
object Shape {
  implicit val shapeRecExtractor: Shape => Rectangle = _.r
}

// insert and delete simplified
val t2 = t0 insert A insert B insert C delete B
assert(t2 == (t0 insert A insert C))

// on the other hand we can do range-queries
val result: Seq[Shape] = t2 search Rectangle(Point(4, 2), Size(1, 3)) // Vector(C)