Elastic/Plastic strain outputs in GEOS

[ryar9534]

The output of mechanical strain in addition to displacements from GEOS has been requested, and I can do/have done some implementation as I have availability.

Output of total strain has been implemented in develop as of PR #3142 .

Next step is the output of a decomposed elastic/plastic strain (really just need one or the other, and the remaining can be recovered with the total strain).

As the constitutive laws for plastic materials have different implementations (and even different theory), I propose we start with some that are perhaps more widely used in our cases for maximum efficiency.

If you would like to request a specific constitutive model or discuss priorities, we can do so here.