Merge branch 'feature/mark-longest-edge-2d-bisection' into 'master'

ref:extensions/dune-alugrid Choose longest edge as initial refinement edge for
2d conforming grids by default

This makes the grid loop free - and hence enables coarsening always, because
the refinement cannot be stuck

See merge request [extensions/dune-alugrid!75]

  [extensions/dune-alugrid!75]: gitlab.dune-project.org/extensions/dune-alugrid/merge_requests/75