Plan data structure refactoring to avoid Rc and RefCell, totally optionnal but nice way to learn

src/space_colonization/space_colonization.rs

@@ -42,6 +42,10 @@ pub struct SpaceColonization {
     ///
     /// If density is 10, then there will be an average distance of 10 between attractors
     density: i32,
+    // TODO learning opportunity : avoid Rc and RefCell to have memory contiguous, increase
+    // performance and simplify code. Using a mutable struct instead of pointers to nodeswill
+    // improve performance. If required, use ids to point to nodes, those ids can be simply their
+    // indices in the array. If the node pointed to is deep, the index can be a vec or a tuple.
     new_nodes: RefCell<Vec<(Rc<Node>, Rc<Node>)>>,
     /// Tree like representation of all nodes
     /// [node: [child1: [grand-child], child2: [grand-child2]]]