DateRange Compare Pane

.. now can drag and drop any date range onto the
   home view compare pane to compatre different
   seasons, and even across athletes.

.. we setup the metrics and measures in the context
   so the charts don't have to, but we still need to
   reference the source context for bests and ridefilecache
   data

NOTE: We need a mechanism for 'locking' source tabs/athletes
      when they are part of a compare to avoid crashing when
      a context is deleted whilst we are comparing

NOTE: We till need a way to remove entries from the compare
      pane, possibly call it when an athlete is closed too
      (see note above).