Cleanup of sine_fit_strain

Take optional arguments, move functions
into functions.

Also autodetect the forces to be plotted
in plot_sine_fit.
User input to approve sine fit data

And give the chance to chop off the ends if that
looks like it will improve things.
Extimate errors on fitting params
Make batch processing use errors
Remove bogus \% and change period error to 2%
Plot the errors properly
Correct error calc on internal friction

Break the calculation down into steps so even
I can do the maths. Also report results
at end.
Errors in plot_youngs_modulis