params["timestep"] may now be a Constant or float.
More fixes for using Constants for params["dt"]
Implement the non-linear divergence term, i.e.

    (eta + depth) * u

Before we used the approximative term

    depth * u

Latter option can be still used by setting the parameter

    params["linear_divergence"] = True

The default for the (Un)SteadyConfigurations is to use nonlinear divergence.