[2p2c,stokesnc,zeroeqnc,multidomain] Add diffusive fluxes to total mass balance

dumux/freeflow/stokesnc/localresidual.hh

dumux/freeflow/stokesnc/model.hh

dumux/freeflow/stokesnc/volumevariables.hh

dumux/multidomain/2cstokes2p2c/localoperator.hh

dumux/porousmediumflow/2p2c/implicit/localresidual.hh

dumux/porousmediumflow/2p2c/implicit/model.hh

test/freeflow/stokes2c/CMakeLists.txt

test/freeflow/stokes2c/stokes2ctestproblem.hh

test/freeflow/zeroeq2c/CMakeLists.txt

test/references/2cnistokes2p2cni-ff-reference.vtu

test/references/2cnistokes2p2cni-pm-reference.vtu

...references/2cnistokes2p2cniboundarylayer-ff-reference.vtu

...references/2cnistokes2p2cniboundarylayer-pm-reference.vtu

test/references/2cnizeroeq2p2cni-ff-reference.vtu

test/references/2cnizeroeq2p2cni-pm-reference.vtu

test/references/2cstokes2p2c-ff-reference.vtu

test/references/2cstokes2p2c-pm-reference.vtu

test/references/2czeroeq2p2c-ff-reference.vtu

test/references/2czeroeq2p2c-pm-reference.vtu

test/references/stokes2c-reference.vtu