Electricity grid

In 0.2.x the game will require an electricity grid:

• Power poles have a square coverage area
• Power poles have a circular maximum wire connection radius
• Wire connection loom is automatic based on pole proximity
• Electric entities must be within a coverage area

As in Factorio a square coverage areas plays well with the build grid, and given the number of powered entities in the game it's a whole lot nicer than manually connecting wires. However the circular connection radius makes the layout of the electricity grid look more realistic by preventing very long diagonal wires, and simplifies things when poles are at different elevations on hills.

Other issues still undecided:

• The Zeppelin supplies 3MW of free electricity so that the early game gets going quickly, and as a fallback to jump-start a base if electricity generation crashes. But how to connect the Zeppelin to the new grid...
  .
• It may be interesting to try and emulate voltage based on a distance to the nearest generator. Encourage breaking up centralised electricity generation, and/or include a transformer pole...
  .
• Belts and tubes consume electricity, but it would be painful to have to cover long belts with poles. Might hand-wave that away, or require the first belt/tube in each line to be powered...
  .
• Other entities could be hard to get to with the grid: offshore Oil Rigs, Laser Turrets on hills, Wind Turbines on hills. Perhaps those can connect directly by also acting as poles...
  .
• Monorail lines ought to act as part of the electricity grid too...
  .

WIP in the electricity branch:
https://github.com/factropy/factropy/tree/electricity