Add Graph::new()

src/models.rs

@@ -26,6 +26,14 @@ pub struct Graph {
 // TODO: Add iterator of edges.
 // TODO: Add iterator of vertex neighbors, see 'fn add_adjacent()'.
 impl Graph {
+    pub fn new() -> Graph {
+        Graph {
+            incidence_headers: vec![],
+            next_incidences: vec![],
+            incidence_vertices: vec![],
+        }
+    }
+
     pub fn vertex_count(&self) -> usize {
         self.incidence_headers.len()
     }