New tag for interface faces

Hello all,

Currently, the face labeling for a `DistributedDiscreteModel` contains a `"boundary"` tag which contains the entities that belong to the physical boundary (i.e the boundary of the global model). The faces belonging to the interface between processors (i.e belonging to local boundary but not to the global boundary) currently belong to the `"interior"` entity.

It would probably be beneficial to be able to distinguish these faces by creating a new entity/tag (`"ghost_boundary"`, `"interface"`, ..?). For instance, when implementing DD methods, patch-based smoothers, etc...

The question would then be how to select the "interface" faces in general for any distributed model (dimensions, conformal/non-conformal,etc...).
Ideas are most welcome!