[freeflow][nonisothermal] Add enthalpy transported by diffusive fluxes

dumux/freeflow/compositional/staggered/fluxvariables.hh

dumux/freeflow/compositional/volumevariables.hh

dumux/freeflow/navierstokes/staggered/localresidual.hh

dumux/freeflow/nonisothermal/localresidual.hh

dumux/freeflow/volumevariables.hh

dumux/material/fluidsystems/h2oair.hh

test/freeflow/ransnc/CMakeLists.txt

test/references/test_channel_lowrekepsilon2cni.vtu

test/references/test_channel_zeroeq2cni.vtu