Leaf face neighors should be returned in SFC-independent order

[holke]

Feature request

Is your feature request related to a problem? Please describe.
If a leaf has multiple leaf face neighbors (i.e. at a hanging face), the order of the neighbors is given in their SFC index order.
This means also that the order of the leaf face neighbors is independent of the face connection's orientation and the actual position in the topology or geometry.

Typically, a solver application wants to match the DOFs of the larger face to the DOFs of the smaller face (i.e. in a Mortar method).
This means that the application has to manually reorder the face neighbors to match their DOF order To do so, the application must know implementation details of the SFC.

If we change the SFC, the order chances and the application has to implement a different reordering.

This leads to:

• SFC dependent high-level code
• Complicated manual reordering methods for each application

Describe the solution or feature you'd like
Reorder the neighbors in the leaf face neighbor method before returning.
I suggest multiple ordering modes:

• SFC order (what already happens)
• Order according to reference coordinates of lower dimensional interface. Let the user pick the main axis of ordering.

For example in the picture below with neighbors A,B,C,D ordering along the x axis of the reference element might yield [A,C,B,D] and ordering along the y axis [A,B,C,D].

Describe alternatives you've considered
keeping as it is, but i describe the drawbacks above.

Estimated priority

"Priority: medium" Should be solved within half a year

Additional context
Add any other context or screenshots about the feature request here.