[Done] Vehicle electrical system improvements

````
Gameplay changes:
    -gasoline/plasma engines no longer have built-in alternators
    -added 3 new alternator part types: motorcycle, car, truck
    -plasma engines produce electricity while running (output comparable to truck alternator)
    -alternators can only be installed on gas engines
    -alternators will slightly reduce engine power available for vehicle speed/acceleration
        -> the bigger alternator (or multiple alternators) or the smaller the engine, the larger the effect.
    -alternator electrical load affects fuel consumption at idle.
        -> a vehicle with many lights, fridges, chargers, etc. will burn more fuel while idling.
        -> idle control allows for practical generator applications: power your workbench with gas instead of solar.
    -added alternators to appropriate pre-defined vehicles

    -gas engines consume small amounts of electricity while running (for ignition system, ECU, etc.)
        -> if alternator fails, gas engines will slowly drain battery (takes a day or more)
        -> gas engines will die if the battery hits 0
        -> exception: small gas engines (1 cyl) don't require electricity to run (they use a magneto instead)

    - starting gas engines will require and consume a small amount of battery (simulates starter motor)
        -> exception: small gas engines (1 cyl) don't require battery to start (they use a pull-start instead)

    -reactors now produce electricity directly, instead of acting as fuel for electric motors
    -reactors only generate electricity on demand, so when your vehicle's electrical requirements are being met by other means (solar or alternators), no plutonium will be consumed
    -reactors can be turned on/off.
        -> install on your solar vehicles for use only at night or in an emergency
    -reactors now have a (very high) power output limit
        -> it is now possible to overload a reactor resulting in an electrical shortage.

Under-the-hood changes:
    -added concept of electrical power 'epower' to vehicles, which is defined in watts
        -> the pre-existing 'power' property can be thought of as mechanical power.
    -all electrical parts now have an epower property
    -vehicle parts can have a positive or negative epower or power
    -alternators convert power to epower (ie. they have a negative power rating and positive epower rating)
    -alternators have a conversion efficiency of 50% to 60%
        -> So installing alternators on electric motors will only result in a net loss of power (just like real life!)
    -electric motors convert epower to power (i.e. they have a positive power rating and a negative epower rating)
    -electric motors have a conversion efficiency of 90% to 95%
    -all electric power accounting (production and consumption) is done in epower, with the final result applied to the batteries.
        -> This allows for small loads (e.g. the vehicle tracker) to have an impact on battery (and therefore fuel) over time.
    -epower sources are used in this order: solar, plasma, alternators(gas), reactors(plutonium), then batteries.
        -> reactors can be turned off and saved for emergencies.
    -engine idle speed controlled by alternator load, which varies based on vehicle epower load.
        -> Higher idle speeds will cause the engine to consume fuel at a higher rate.
    -fuel consumption is now also more granular.
        -> Lower fuel consumption at idle is now possible.

Other changes/fixes:
    -added caches for alternators, engines, solar_panels, and reactors
        -> should make the game run faster in or around vehicles
    -made better use of existing fuel tank cache.
    -refresh caches on vehicle load.
    -added small storage battery to bubble car (acts as a buffer between the reactor and electic motor)
    -added missing car battery to cube van
    -added missing car battery to hippy van

edit: updated to reflect latest commit
````