Add quadtree nearestToPoint, and Readwrite locking (#217)

orx-quadtree/src/commonMain/kotlin/IQuadtree.kt

@@ -0,0 +1,45 @@
+package org.openrndr.extra.quadtree
+
+import org.openrndr.math.Vector2
+
+interface IQuadtree<T> {
+    /**
+     * Clears the whole tree
+     */
+    fun clear()
+
+    /**
+     * Finds the nearest and neighbouring objects within a radius
+     * (needs to have a different name so there is no ambiguity when the generic object type is Vector2)
+     *
+     * @param element
+     * @param radius
+     * @return
+     */
+    fun nearestToPoint(point: Vector2, radius: Double): QuadtreeQuery<T>?
+
+    /**
+     * Finds the nearest and neighbouring points within a radius
+     *
+     * @param element
+     * @param radius
+     * @return
+     */
+    fun nearest(element: T, radius: Double): QuadtreeQuery<T>?
+
+    /**
+     * Inserts the element in the appropriate node
+     *
+     * @param element
+     * @return
+     */
+    fun insert(element: T): Boolean
+
+    /**
+     * Finds which node the element is within (but not necessarily belonging to)
+     *
+     * @param element
+     * @return
+     */
+    fun findNode(element: T): Quadtree<T>?
+}