Delta Watts, Torque, Cadence and HR

.. Following on from the recent update to add acceleration
   this update adds other derived data series based upon the
   rate of change.

.. Added to the ride plot and the CP plot.

Not sure of the overall utility of these updates but bear in
mind that they are targetting sprinting and track users and
analysis.

As well as the positive side of this (development of power
cadence etc) we also want to think about and collect data
on fatigue rate (possibly only power and torque)
- fatigue over time
- fatigue over pedal stroke