[facet][box][cm] fix context for Dirichlet coupling

Until now in the box-facet coupling framework, "interpolated" vol vars were stored in the context for the coupling faces of the bulk domain. However, if Dirichlet coupling is used, this gives wrong results, as then the wrong values are used as Dirichlet conditions for the vertices of the bulk grid.

This was not seen in the pressure convergence behavior, but it can be seen when looking at the profiles of the transfer fluxes, which show oscillatory behavior.

This commit introduces that for Dirichlet coupling, the values at the vertices are used for generating the context. This completely removes the oscillatory transfer fluxes. In the new implementation, whereever a bulk context is built or modified, we first determine if a face is tagged to couple via Neumann or Dirichlet-type boundary conditions.