Electric bikes & muscle power [WIP]

Muscle engines (pedals, hand rims) can have alternators attached. Only one muscle engine can be installed on a vehicle. This can be in combination with any number of regular engines. 

There is a max weight that a muscle engine can move, which is based on strength. Over this, the vehicle won't accelerate unless another engine is helping.

Changes:
Adds electric bike
Adds medium storage battery

Can install alternators on mechanical/muscle engines
Charges batteries while idling as well
Can install only one muscle engine, with any amount of regular engines
Remove PEDALS, HAND_RIMS, PADDLES flags from game
Muscle engine install not more difficult due to already installed engines
Move fuel coefficients to class level, underneath other fuel definitions
Now broken engines show in engine toggle list
If the player is in control of a vehicle and engine is off, toggling an engine will start the engine again. This prevents weirdness when running out of fuel etc
Can now 'stop driving' when the vehicle has only mechanical engines active
Using muscle engines costs fatigue, hunger and thirst, proportional to strain on the 'engine'

TODO:
- [x] Add medium storage battery to spawn pool
- [x] Add electric bike to spawn pool
- [x] Greater cost if over safe speed on muscle engines
- [x] Check strength limit works
That code is strange, at least.

Not storing the result of `veh_at`, if the player is not on a vehicle, `part_under_player` is left uninitialized by `map::veh_at`. The variable is than used, leading to undefined behavior (the debug builds "could" add automatic initialization to 0, which would be a valid part index, the debug build might not fail).
However, `vehicle::part_with_feature` will not work and crash with an invalid part index.

I suggest checking the result of veh_at: `if(veh_at(...) == this) {...`, the part index would than be valid for the `this` vehicle.