if search braking power finds a value <0 => overload response. driver strategy has to find another way to proceed...
(can happen if the gear is disengaged and the requested deceleration can not be reached, i.e. the deceleration is higher)

gearbox disengages when braking and engine-speed would drop below idle speed

- introduce driver info about desired behavior

- open the clutch if the vehicle's speed gets too low and the engine speed goes below idle speed

consider the case that 'nextTargetSpeed != 0' for computing the next simulation distance

make vehicle code testable, test for airdrag resistance, adapt componentdata files to new file format

Additional property in mission to disinguish which CdA Value to use (Rigid Truck, Longhaul Cycle w/ trailer)

corrected gear shift behaviour when engineSpeed near the clutch-slipping threshold (truck_drive_20_downhill_15, truck_acc_20_60_down, truck_dec_60_20_down)