WIP [mpfa][fickslaw] use correct flux sign switch query

Closed Dennis Gläser requested to merge fix/fickslaw-mpfa into master

The advection-related query if the sign of a flux has to be flipped was used in ficks law. This leads to compiler errors in the context of models that consider diffusion but no advection. Also, it would have introduced wrong flux signs in mpfa schemes in which the interaction volume regions depend on cell parameters (e.g. mpfa-l).

Edited by Dennis Gläser

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