Merge branch 'remove-obsolete-amglevel-access' into 'master'

ref:staging/dune-uggrid The UG3 multigrid data structure used to have an array
'amggrids' with grid levels for AMG. That array is long gone, but it was still
overwritten with nullptrs in the multigrid setup code. A serious memory bug,
and I am amazed that nobody ever observed any effects.

See merge request [!145]

  [!145]: gitlab.dune-project.org/staging/dune-uggrid/merge_requests/145