Add Power 'heat' to RideFileCache

.. to indicate which power-durations are getting worked hardest

.. Calculate a 'heat' score for the power duration curve.
   A score that counts how many rides in the aggregate
   have a power-duration best that is within 10% of the
   best value calculated.

.. next step is to add it to the CP curve (when showing
   power) to provide some kind of shade/heat indicator
   for the area of the curve.