Changed the way stiffness is calculated by the InstronAnalysis class
Previously it used 50% and 100% of max force, now it used 25% and 75%

Included a zeroing of the displacement in the DAQInstron class when gain
is applied. This didn't affect the quasi-static tests as the instron was always
zeroed before start, but was an issue when analyzing the de Bakker tests.

Updated the Specimen constructor help.