Strain calulculation in µDIC

[PolymerGuy]

Continuing from here: https://github.com/PolymerGuy/muDIC/issues/24#issuecomment-624749895.

For the sake of future development, we should decide on a baseline for how the strains are calculated in µDIC.

I propose the methodology found in this document: Strain_calulations_in_DIC.pdf

The plan is to later merge it into the documentation.

I am working on code which strictly follows this methodology and uses a minimum of "back and forth" calculations, but due to time constraints, it's not ready for review yet.

@illyasst and @diehlpk, I would love your feedback on this!

[diehlpk]

@PolymerGuy I would suggest using the Green-Lagrange strain tensor as the default, since this is the most common in the applied mechanics community.

You might consider adding a YAML configuration file to define the favorite settings.