Cleanup/box facet dof enrichment

What this MR does / why does DuMux need it:

The box-facet-coupling model requires duplication of dofs in the bulk domain at faces coinciding with fractures in order to capture potential jumps in the solution across the fractures. At fracture tips ending inside the bulk domain, or on fractures that coincide with the bulk domain boundary, no duplication occurs. However, at fractures tips located at the bulk domain boundary, duplication does occur.

One case that wasn't handled properly yet is what's shown in the pictures in the comments below. This MR fixes that.

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