[freeflow] Fix test and cleanup

dumux/discretization/staggered/freeflow/fickslaw.hh

dumux/freeflow/navierstokes/staggered/fluxvariables.hh

dumux/freeflow/rans/problem.hh

dumux/freeflow/rans/twoeq/lowrekepsilon/model.hh

dumux/freeflow/rans/twoeq/lowrekepsilon/problem.hh

...eflow/rans/twoeq/lowrekepsilon/staggered/fluxvariables.hh

...eflow/rans/twoeq/lowrekepsilon/staggered/localresidual.hh

test/freeflow/rans/CMakeLists.txt

test/freeflow/rans/test_pipe_laufer.input

test/references/pipe_laufer_lowrekepsilon.vtu

test/references/test_channel_zeroeq2c.vtu

test/references/test_channel_zeroeq2cni.vtu