Externalize the power model from the turbo trainer driver

[jovial]

Initial thoughts:

• TurboTrainer implementation should become observer of InstantaneousRideParameters (wind speed, gradient etc)
• PowerModel also an observer of InstantaneousRideParameters
• TurboTrainer should advertise the data it provides i.e power and speed. Should prefer power data; from this we can calculate virtual speed and therefore distance. If turbo only provides speed, integrate this for distance.
• TurboTrainer could be an obvserver of virtual speed i.e so we can use PID internally. Alternatively, externailze the PID controller (TurboTrainer interface would require a resistance scale 0-100%).