[cleanup][shallowwater] Cleanup test and model

dumux/flux/shallowwaterflux.hh

dumux/freeflow/shallowwater/boundaryfluxes.hh

dumux/freeflow/shallowwater/fluxvariables.hh

dumux/freeflow/shallowwater/indices.hh

dumux/freeflow/shallowwater/iofields.hh

dumux/freeflow/shallowwater/localresidual.hh

dumux/freeflow/shallowwater/model.hh

dumux/freeflow/shallowwater/problem.hh

dumux/freeflow/shallowwater/volumevariables.hh

test/freeflow/shallowwater/dambreak/main.cc

test/freeflow/shallowwater/dambreak/problem.hh

test/freeflow/shallowwater/dambreak/spatialparams.hh