[freeflow][nonisothermal] Add output of thermal conductivity

dumux/freeflow/nonisothermal/vtkoutputfields.hh

test/references/pipe_zeroeqni.vtu

test/references/stokes2cni-advection.vtu

test/references/stokes2cni-diffusion.vtu

test/references/stokesni-conduction-reference.vtu

test/references/stokesni-convection-reference.vtu

test/references/test_channel_lowrekepsilon2cni.vtu

test/references/test_channel_zeroeq2cni.vtu