mailing list as maintainer

As Steffen steps down as the maintainer of dune-typetree and nobody claims this his or her project, I propose to set the Dune developer mailing list as the dune.module email address. Further, there will be no single maintainer to be asked for approval. Typetree will be like the core module, were everybody does as he thinks it is the best. We help each other by discussions and comments. I assume that especially users of dune-functions and PDELab will follow the development and contribute further improvements or features.

@smuething @dominic @oliver.sander @carsten.graeser @christi Would that be ok?