We should discuss how to work with flow graphs and lookups, based on #123 (comment).

This includes how to to handle common lookups for data flow variables, data flow characteristics (e.g. characteristic values regardless of data flow variable name) and node characteristics.

Additionally, there are some pitfalls posted by @Entenwilli:

• Copying a PartialFlowGraph is discouraged (References need to be updated internally)
• Evaluating a PartialFlowGraph standalone is possible in PCM, but not with DFDs
• By proxy, flow graphs are shallow copied, so they reference the same PartialFlowGraph after copy

This discussion is especially valuable regarding analysis extensions.