Implement LvCoordinator

[ckittl]

Implement the actor, that does the coordination for the low voltage level within an area of interest. For details refer to https://osmogrid.readthedocs.io/en/latest/concept.html and the documented protocol proposal.

• Implement messages, understood by the actor
• Implement logic, reacting on messages
• Determine data and configuration needs and eventually compile another issues from that
• Write tests