[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

Fixes #692 (closed)