[md][stokesdarcy] Upwinding of component enthalpy is probably wrong

const bool insideDiffFluxIsUpstream = std::signbit(diffusiveFlux[domainICompIdx]);

should probably be

const bool insideDiffFluxIsUpstream = diffusiveFlux[domainICompIdx] > 0.0;

• Furthermore, we should check if all fluidsystems implement componentEnthalpy.