Merge branch 'feature/parametertreeconstructor' into 'master'
ref:core/dune-istl This MR Only contains the ParameterTree constructors from
MR [!85].
See merge request [!315]
[!85]: gitlab.dune-project.org/NoneNone/merge_requests/85
[!315]: gitlab.dune-project.org/core/dune-istl/merge_requests/315