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.