Matching the face to the boundary conditions on the coarse mesh

[HenningScheufler]

Thanks for the maintaining and developing this extremely useful library.

The documentation is quite good and i was able to get started quite quickly. However, it unclear to me how i can correlate a face of a cell to a boundary condition on the coarse mesh. So i want to basically know which cell face has which boundary label (e.g. top,bot,left,right):