[!274] Feature/amg memory management

Merge branch 'feature/amg-memory-management' into 'master'

ref:core/dune-istl This branch is cleaning up the AMG memory management in
order to get rid of manual pointer juggling and thus improve maintainability.

See merge request [!274]

  [!274]: gitlab.dune-project.org/core/dune-istl/merge_requests/274