Extend leaf_face_neighbor function to ghost elements

[holke]

Feature request

Is your feature request related to a problem? Please describe.
For some applications it is required to compute the face neighbors of ghost elements (if they are local element or ghosts).
Currently our lfn function does not support ghost elements as input.

Describe the solution or feature you'd like
Extend the lfn function to accept ghost elements as input parameters.
Return only those neighbors that are local elements or ghosts.

Describe alternatives you've considered
n/a

Estimated priority

"Priority: high" Should be solved as soon as possible

I need this currently.

Additional context
I started working on this. Mostly created the issue to inform other about the existing plans.