Potential bug: no health workers in user_graph

[dburov190]

In ContactNetwork.from_networkx_graph, which is used for building the user_network form network, the set of health workers is set to empty. I'm not sure if this an issue or not; after all, we only need to know which nodes are health workers in kinetic simulations, right? and that thing uses network, so it does know which nodes are health workers.

Still, it'd be nice to trace all the places user_network is used and make sure it's okay.