Prefs pane sizing to font size

.. to stop it being too small when you select a larger font.
Bests offset error

.. when plotting 1s best aka Max Power, the code was using
   the wrong offset (out by one) and returning 0w.
Cache StressCalculator in LTMPlot

A bit of a compromise since really we should move the
stress calculators to the Athlete class and reuse when
no filtering is applied.

This code fix just means the SC is not recalculated for
each of the individual curves (LTS, STS etc) and pretty
much halves (or better) the time taken to refresh the
LTMPlot.

For those with very few rides (<500) this isn't much of
an issue, but for some with lots (2000 or more) it can
be quite tedious.
Fix find intervals no gps/speed bug

.. flawed logic mean't that findBests didn't work
   when distance is not present via speed or gps.