Merge branch 'feature/use-uggrid' into 'master'

instead of dune-alugrid which is licensed under GPL2, we use dune-uggrid as
the default grid manager, which is licensed unter LGPL-2.1

See merge request [duneuro/duneuro!34]

  [duneuro/duneuro!34]: gitlab.dune-project.org/duneuro/duneuro/merge_requests/34