[3p3c][localresidual] fix calculation of diffusive fluxes

Sum of fluxes is flux of the remaining phase only in the right reference system. That is why this has to be calculated and then the units for all fluxes need to be adapted.

@holle this could be the reason for the unphysical behaviour of the remediation test. If not I still think this is the correcter version. I still need to test if it changes tests.