Implementation incomplete for isotropic linear poroelasticity bulk rheology with reference stress/strain

[baagaard-usgs]

@rwalkerlewis

Description

The point-wise functions for stress that are supposed to account for reference stress and strain (f1u_refstate()) create variables for the reference stress and strain but do not use them in the calculation of stress.

Requested resolution

Finish implementing f1u_refstate() and add an appropriate test that makes use of this feature.