[core] Adds a .copy() method to graph structure.

bonobo/structs/graphs.py

@@ -1,3 +1,5 @@
+from copy import copy
+
 from bonobo.constants import BEGIN
 
 
@@ -62,6 +64,15 @@ class Graph:
 
         return self
 
+    def copy(self):
+        g = Graph()
+
+        g.edges = copy(self.edges)
+        g.named = copy(self.named)
+        g.nodes = copy(self.nodes)
+
+        return g
+
     @property
     def topologically_sorted_indexes(self):
         """Iterate in topological order, based on networkx's topological_sort() function.