[rans] Change averaging methods for the two-eq models

...x/freeflow/rans/twoeq/kepsilon/staggered/fluxvariables.hh

dumux/freeflow/rans/twoeq/komega/staggered/fluxvariables.hh

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

test/freeflow/rans/CMakeLists.txt

test/freeflow/rans/test_pipe_laufer.input

test/references/pipe_laufer_komega.vtu

test/references/pipe_laufer_lowrekepsilon.vtu

test/references/test_flatplate_lowrekepsilon2cni.vtu