feat: plastic strain output

[ryar9534]

Introducing a plasticStrain output field in addition to the current total strain. This is done by subtracting the elastic strain (obtained via the getElasticStrain method of the constitutive laws) from the total strain and averaging over quadrature points.

Rebaselining will be required because of the new field.

Ideally, plastic strain would only be output when a plasticity model is being used, but Im not sure how exactly we will do that...

I will run the Drucker-Prager and Modified Cam Clay wellbore problem to test