Fix/facet coupling on boundary

What this MR does / why does DuMux need it:

The facet-coupling framework seems to not have worked for the case of lower-dimensional domains coinciding with the bulk domain boundary, as was reported to me in private by an external user. In particular, I observed that:

• the box scheme led to wrong results when Dirichlet boundary conditions were specified at the boundaries coinciding with a lower-dimensional domain (the Dirichlet values were set and the residual was thrown away)
• the specialization of evalFluxes in the CCFacetCouplingLocalResidual was actually not called (at least with my compiler), but the one of the parent class -> leading to wrong treatment at boundaries that are "interior" and "exterior" at the same time...
• for mpfa, the face count was wrong in interaction volumes that have "interior boundary" faces that are "exterior" ones at the same time.

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