[staggered][io] Enforce the use of decltype(partSol) as template argument

* when passing the result of partial(sol) to the staggered vtkWriter,
  one has to make sure the type of the sol vec (Dune::MultitypeBlockVector
  storing references) passed to the constructor
  is the same as the one used as template argument
* failing to do so results in a segfault
* this commit ensures the correct types at compile time