Periodic Boundary Conditions for FreeFlow

Feature request

Periodic Boundary Conditions for FreeFlow

What does this feature / why does DuMux need it:

Periodic Boundary Conditions are a common and useful BC and help to overcome the problem of finding appropriate BC for finite domains.

Which issue does this feature fix (if any)

It would enable using Periodic Boundary Conditions in a coupled Darcy-FreeFlow model. Periodic Boundary Conditions are already implemented for Darcy's law but can't be used reasonable in a coupled model with freeflow, because freeflow doesn't allow Periodic Boundary Conditions.

Anything else we need to know?:

Already mentioned in #487 (closed)