Add UnpackTypeList_t, UniqueTypeList_t, and uniqueTypeList()

Add some tiny but handy utilities for Dune::TypeList.

• The template alias UnpackTypeList_t unpacks a Dune::TypeList into a varaidic template. This e.g. allows to do a Dune::TypeList to std::tuple conversion using UnpackTypeList_t<std::tuple, Dune::TypeList<T...>>
• The template alias UniqueTypeList_t makes the entries in a Dune::TypeList unique and is an alias for the resulting Dune::TypeList
• The uniqueTypeList() functions makes the entries in a Dune::TypeList unique and returns the resulting Dune::TypeList.