code cleanups

examples/uopa_graph.rs

-use opossum::nodes::NodeDummy;
-use opossum::optic_node::OpticNode;
 use opossum::optic_scenery::OpticScenery;
+use opossum::nodes::NodeDummy;
 
 use std::fs::File;
 use std::io::Write;

src/optic_ports.rs

 use crate::error::OpossumError;
-use std::{collections::HashSet, mem::swap};
+use std::collections::HashSet;
 
 #[derive(Default, Debug)]
 pub struct OpticPorts {

src/optic_scenery.rs

 use crate::error::OpossumError;
 use crate::optic_node::{OpticNode, Optical};
 use petgraph::algo::*;
-use petgraph::data::Build;
 use petgraph::prelude::{DiGraph, EdgeIndex, NodeIndex};
 
 type Result<T> = std::result::Result<T, OpossumError>;