A shortest-path algorithm for power manager to indicate whether a building is powered or unpowered

[iaGuoZhi]

when a building has no power or water it is „unhappy“. Residents will move out, property value lowers and eventually it will get abandoned.
But it will stay there (and later display a symbol that it’s unpowered)
Police / hospital / … will stop working
But nothing of that is implemented. Right now, there should only be a bool in mapnode to determine if a node is powered or not
So for now the task would be:
When power consumption > production use the algorithm to determine which buildings are powered and set their entire mapnode to powered or unpowered
If there’s enough power we won’t need to use the algorithm, all are powered

Originally posted by @SimplyLiz in #879 (comment)