update stokers with latest estimated state

[robberwick]

A stab at updating the stokers with the latest estimated state, using the Observer pattern.

Things of note:

• introduces Subject and Observer interfaces
• StateEstimator implements Subject, maintains list of observers
• Stoker implements Observer
• StateEstimator notifies all observers when new state is estimated
• introduced a MOTOR_POSITION enum, and updated MotorSpeeds to use it
• Stoker has a position property that it uses to pull the correct value from the new state payload when updated

I didn't go the whole hog and replace the StateEstimator updating with the command pattern. It seemed like unnecessary noise at this point, as this branch was already getting a bit busy.

addresses #29

[markmellors]

tested again - driving works as before. Wheel speeds and steering still work, odometry related prints look sane.

[robberwick]

If we're happy that this is ok, let's get it merged so that we can rebase https://github.com/thingswebuilt/osod24_firmware/pull/40 on it and merge that too (assuming we're happy with that also)