Altitude control

[Thonners]

Implement the HelicopterPilot responding to changes in throttle demand

• Research what the normal input on the RC controller is for throttle
• Code it up!

[Thonners]

Left stick up/down apparently normally controls both the 'collective pitch' and the throttle - the latter needs to be increased to keep a constant head speed when the angle of attack of the rotor is increased (which is what happens when the collective pitch moves). We therefore also need to have a map for blending throttle increases and collective pitch increases

Finally, there's normally also an extra mode ('idle up') which keeps the motor speed high but allows down on the stick to make the collective pitch negative, making downforce and therefore allowing inverted flight. So we need this extra mode too for inverted flight. Perhaps later