WIP: TransferOperatorAssembler assembles transfer matrices with wrong dimensions.
Note: Currently the dune-fufem tests fail due to an unrelated issue.
e34a56e7 introduces a test that should fail when testing ALUGrid without SubGrid due to a possible bug in dune-alugrid. It stems from the fact that leaf grid vertex indices do not seem to be reliable. A related issue is due for dune-alugrid: https://gitlab.dune-project.org/extensions/dune-alugrid/issues/28
A workaround seemingly was already found by @babushkina, but might need some further investigation.