Entropy stabilization when shearing

[jdannberg]

We apply standard entropy viscosity to a shear bands test and see oscillations where the velocity is small. This is because the stabilization is scaled with the maximum velocity in a cell, and does not see shearing.
@tjhei suggests to use max(max_velocity, strain_rate*h).

[bangerth]

This is a duplicate of #293.

[tjhei]

Not quite, because we have a nonzero but small velocity in every cell.

[bangerth]

But aren't these both symptoms of the same underlying cause?