Simplify power system

[darozak]

I want to redesign and simplify the power system as follows:

• Batteries loose power through use and cannot be recharged.
• Generators produce the same amount of power every turn but cannot store it.
• No longer any need for maxPower because batteries cannot be recharged.

[darozak]

I've decided to allow for rechargeable batteries. This will mean that robots might need to rest up before advancing.

[darozak]

I'll need to make a generatePower routine that adds power to batteries at a certain rate based on the amount of time that has passed since the robot's last turn.