Flexible steinberger viscosity boundaries

Added more possibilities to calculate the viscosity in the steinberger material model. Made the viscosity cutoff values input parameters for higher flexibility. I need this for comparisons to other (CitcomS) models with slightly different viscosity models, but it includes at least one generally useful feature. Now users can decide whether to use the lateral average temperature or the adiabatic temperature as reference temperature, at which the thermal viscosity variation should be zero. This is especially of importance for long model runs, where the laterally averaged temperature may be different than the adiabatic temperature profile calculated at the beginning. Also added tests for the viscosity calculations.