New Metric Fatigue Index

Looks at the max and min (non-zero) power values and
uses the difference between them to calculate a fatigue
index as a percentage.

i.e.
FI = (maxP - minP) / maxP * 100.00;

This is really only useful for targetted intervals as for
most riding there will always be a period of time where
the rider coasts or takes it easy.

We may look to improve it by smoothing or comparing to the
average power instead of minimum power.
Fix 2d crash

Qwt 6.1 seems to delete objects when they are detached from
the plots by default, so we don't delete them after detach
any more in ScatterPlot.