Support subdomains in InterfaceIterator

[termi-official]

Currently the InterfaceIterator just supports iteration over the full domain.

[KnutAM]

To be solved by solving #850?

[termi-official]

Can be resolved together. However, the issue is actually independent, as it is also possible to have subdomains without mixed grids.

[KnutAM]

Can be resolved together. However, the issue is actually independent, as it is also possible to have subdomains without mixed grids.

True - just put the remark here since the design should probably account for that possibility as well, and I think the "Ferrite"-way of mixed grids will require subdomains even if the reverse is not true as you say.