[kepsilon] Improve switching threshold

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

test/freeflow/rans/CMakeLists.txt

test/freeflow/rans/test_pipe_laufer.input

test/freeflow/rans/test_pipe_laufer_reference.input

...eeflow/rans/test_pipe_laufer_reference_wallfunction.input

test/freeflow/ransnc/CMakeLists.txt

test/references/pipe_laufer_kepsilon.vtu

test/references/test_flatplate_kepsilon2cni.vtu