Add design patterns for larger models

[hellemo]

I think it might be a good idea to include some tips on how to build larger models and use functions to generate similar nodes or areas etc to get models that are easier to work with. Just building one some existing examples will give you a large array of nodes for example, that is not easy to work with.

Some ideas:

• functions to generate nodes/areas/subsystems
• use dictionaries to index nodes/edges