Fix non linear nnewt viscosity

For non-Newtonian Stokes flow simulations, the viscosity depends on the second invariant of the strain-rate tensor. It was no previously possible to update this field inside the non-linear iteration loop, which lead to inconsistencies.

This branch fixes that by adding a (magically) named field "StrainRateSecondInvariant", which, if present, is updated before the viscosity field, inside every non-linear iteration.

A multi-material slab detachment benchmark case (first time-step only) has been added to test this new functionality.