Draft: Periodicity with Subgrid

What this MR does / why does DuMux need it:

DUNE Subgrid currently does not preserve intersection.boundary() from the host grid, which we use to detect periodic boundaries with intersection.neighbor() == intersection.boundary() == true (see https://gitlab.dune-project.org/extensions/dune-subgrid/-/issues/8).

This MR extracts the logic for detecting periodic intersections from discretization-specific gridgeometries to a helper struct. This could also hold information regarding grid capabilities for periodicity, which is currently more spread out, e.g. in the freeflow velocitygradients.hh.

Notes for the reviewer

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