Plot Peak Power et al 2 of 2

Updated RideFileCache and LTMWindow/Plot to
plot the user defined durations for peak power.

Actually we support all the different meanmax
measures available in the CP chart, so you can
now track peaks for user defined durations of;

            * Power
            * Watts per Kg
            * xPower
            * NP
            * Heartrate
            * Speed
            * Cadence
            * Torque
            * VAM

I have not spent much time optimising for performance
but reusing the same 'seek' approach used by the recent
update for best() and tiz() functions in the datafilter.

Fixes #322.