deployment generation assumes symmetrical links

[rmm-fcul]

Messages are not delivered if a path from receiver -> sender does not exist in nbg. (despite the presence of a sender->receiver pathway).

The deploy.prepare stage should either

• warn the user that the configuration will not work; OR
• insert the reverse edge automatically when absent.

I favour the warning/(fatal) error, since the labelling of reverse is not at all obvious how to do generally: they need to be unique (see #68).