Simple Batteries

[Indy2222]

As a step towards initial support for energy in the game, lets implement batteries:

• Every unit & building has a battery. The battery has maximum capacity in Joules and current energy level in Joules.
• The battery discharges at a constant rate (a constant in the code).
• Movement of unit is blocked once the energy goes to 0.
• Attacking is blocked once the energy goes below X (a constant in the code).
• Manufacturing is blocked once the energy goes below Y (a constant in the code).

Note that charging is not handled as part of this issue.