Output investment graphs as DOT files

We currently have a feature to output commodity graphs, but we also create _investment graphs_ which are slightly different:
- only primary flows
- only SED/SVD commodities
- edge labels don't matter
- duplicate edges can be compressed
- strongly connected components are compressed into "supernodes"
- nodes are grouped into layers

It would be useful to have a way of visualising these investment graphs so the investment order can be understood.

This should probably be a hidden developer feature for now