Removal of dune-common/dune/function.hh makes TNNMG unusable

In https://gitlab.dune-project.org/core/dune-common/-/commit/cc5bb9fda147168ef83d88c1813bc43a1dc1277c the deprecated file with the interfaces Dune::Function and Dune::VirtualFunction is now deleted.

dune-tnnmg still depends on this file in the definition of the functional classes.

I am opening an issue since I am not sure how severe the problem is.

A workaround would be to transfer the old interface from the dune-common module to dune-tnnmg, since we only need a few members and methods.