Implement LvRegionGenerator

[ckittl]

Implement the actor, that takes care of generating the low voltage grid level within a distinct regional area. Here, it is targeted, that it takes care of all grids within a single municipality. 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