Add @value property

Author: rocky

pymathics/graph/__main__.py

@@ -463,6 +463,10 @@ def get_sort_key(self, pattern_sort=False):
         else:
             return hash(self)
 
+    @property
+    def value(self):
+        return self.G
+
     @property
     def vertices(self):
         return self.G.nodes