resolve disconnected network handling

[atravitz]

Currently, Konnektor doesn't allow for disconnected subgraphs to exist within a network object. However, OpenFE does. We need to resolve this discrepancy before integrating Konnektor into OpenFE.

I believe we can keep both of these behaviors, and that Konnektor can just have more strict requirements than OpenFE. This might be too optimistic though ...

see here for context: #927 (comment)