GitLab

Created by: maypink

Refactor Graph (& OptGraph) usages:

• Now graph.operator can not be used outside of graph
• Remove direct usaged of graph.nodes: now there is a private field _nodes and property nodes. However users can still modify nodes using this property
• Some methods (sort_nodes and actualise_old_node_children) were not moved to graph interface since they are not in use anywhere