Add (so far non-fucntional) reference node.

examples/pa_doublepass_graph.rs

-use opossum::nodes::NodeDummy;
+use opossum::nodes::{NodeDummy, NodeReference};
 use opossum::optic_node::OpticNode;
 use opossum::optic_scenery::OpticScenery;
 
@@ -13,6 +13,9 @@ fn main() {
     let n3 = scenery.add_node(OpticNode::new("Faraday", Box::new(NodeDummy)));
     let n4 = scenery.add_node(OpticNode::new("0° mirror", Box::new(NodeDummy)));
 
+
+    //let ref_node= NodeReference::new(&Box::new(NodeDummy));
+
     let mut node= OpticNode::new("Faraday", Box::new(NodeDummy));
     node.set_inverted(true);
     let n3i=scenery.add_node(node);

src/nodes/mod.rs

 //! This module contains the concrete node types (lenses, filters, etc...)
 mod node_dummy;
+mod node_reference;
 
-pub use node_dummy::NodeDummy;
\ No newline at end of file
+pub use node_dummy::NodeDummy;
+pub use node_reference::NodeReference;
\ No newline at end of file

src/nodes/node_reference.rs

+use crate::optic_node::{OpticNode, Optical};
+
+/// A virtual component referring to another existing component. This node type is necessary in order to model resonators (loops) or double-pass systems.
+pub struct NodeReference<'a> {
+    reference: &'a Box<dyn Optical>,
+}
+
+impl<'a> NodeReference<'a> {
+    pub fn new(node: &'a Box<dyn Optical>) -> Self {
+        Self { reference: node }
+    }
+}
+
+impl<'a> Optical for NodeReference<'a> {
+    /// Returns "dummy" as node type.
+    fn node_type(&self) -> &str {
+        "reference"
+    }
+}