Merge branch 'cleanup/staggered-energy' into 'feature/staggered-energy'

dumux/freeflow/staggered/model.hh

dumux/freeflow/staggeredni/properties.hh

dumux/freeflow/staggeredni/propertydefaults.hh

test/freeflow/CMakeLists.txt

test/freeflow/staggered/CMakeLists.txt

test/freeflow/staggered/channeltestproblem.hh

...freeflow/staggered/test_channel_stokesni_conduction.input

...freeflow/staggered/test_channel_stokesni_convection.input

test/freeflow/staggeredni/CMakeLists.txt

test/freeflow/staggeredni/channeltestproblem.hh

test/freeflow/staggeredni/diffusiontestproblem.hh

test/freeflow/staggeredni/test_channel_ni.cc

test/freeflow/staggeredni/test_diffusion_ni.cc

test/references/stokesni-conduction-reference.vtu

test/references/stokesni-convection-reference.vtu