Add example 8 test case images.
Add example 8 test script.
Add measure reference methods.

export_sizes() now writes information about the reference measure object
to file. A method get_mesaure_size() for grabbing the measure object's
pixel dimensions was also added.
Merge remote-tracking branch 'origin/measure-calibration' into mult-compression
Add export of spring constant.

The spring constant is calculated for the convenience of the user if
force values for compression images were provided.