Remove check of number of equations.

Since the shallowwater viscous momentum flux is only implemented for 2D
depth averaged models, the ShallowwaterViscousFlux checks if the number
of equations is euqal to three. Unfortunatly, this prevents the use of
the viscous flux for models which couple shallowwater flow and sediment
transport, which have more equations.
Status Name Job ID Coverage
manual full-dune-2.7-clang
manual full-dune-2.7-gcc
manual full-dune-master-gcc
manual minimal-dune-2.7-gcc
  Downstream Modules
created trigger lecture