[!691] Fix activation of union members

Merge branch 'feature/proper-union-activation' into 'master'

ref:core/dune-common

-   Members of a union need to be activated by calling a constructor (e.g.
    using placement new)
-   This allows to also delete the move-assignment operators such that we can
    now better protect the internal classes against misuse.

See merge request [core/dune-common!691]

  [core/dune-common!691]: gitlab.dune-project.org/core/dune-common/merge_requests/691