Abstract this into a general purpose tool that converts networkx graphs into LP problems

Might need to establish converters/support for different classes of problems separately. Worth investigating at least. Problems to target:

* multi-commodity network flow (current implementation)
* Scheduling (i.e. the "volunteer coordinator" problem)
* maybe get some inspiration from the PuLP docs

Start by locating the current multi-commodity network-flow implementation and tracing how it converts networkx graphs into LP problems. Review the PuLP documentation for relevant patterns, then define separate converter support for network flow and scheduling; the work is done when both targeted problem classes are supported by a general-purpose tool.

Written by the indexing model from the issue text.