Test viscoplastic material model

[bobmyhill]

#5982 revealed that the viscoplastic model needs some additional testing for the various switches and choices.

One possibility is to set up a prm with an increasing strain rate from left to right through the domain that spans the viscous->plastic transition, outputting the viscosity over several timesteps. Different prms could be set up with the different options.

An monotonically decreasing viscosity (without elasticity active) would act as a basic sanity check.

With elasticity active, the prm could be run over several timesteps to approach steady state.