Merge pull request #865 from Walibu/master

Power value from Suunto .gpx file and fix with unvalid 0m Altitude in .tcx
Fix comments in RideFileCache::meanMaxPowerFor()
MetricAggregator Model Refresh (Part 1 of 3)

.. added the code to aggregate power data when deriving
   model parameter history over time.

.. need to get a class that applies the various models to
   generate the parameters we need (so we can reuse) in
   part 2.

.. then need to store and turn into a metric of some kind
   in part 3.

.. this is largely a checkpoint commit to enable cross platform
   testing with Mac and Windows.