Quadratic Least Squares Trend Line

Added a quadratic least squares trend line for the LTMplot
as the linear regression was generally too blunt.

This is a hack to get the code into the repo -- will know work
on refining the LTMPlot settings to enable users to specify
the kind of trend line they want.

The new trend line might also be useful for other curve fitting
functions (e.g. realtime virtual power curve, a peak power chart
by cadence/pedal speed).