[staggered] Passing the object of partial(sol) to VtkWriter leads to segfault

When using the staggered scheme in a multi-domain context, initializing the VtkWriter like

auto partialSol = partial(sol);

StaggeredVtkOutputModule<GridVariables, 
                         GetPropType<TypeTag, Properties::SolutionVector>> vtkWriter(*gridVariables, partialSol, name);

leads to a segfault.

If the writer is instead initialized like

auto partialSol = partial(sol);

StaggeredVtkOutputModule<GridVariables, 
                         decltype(partialSol)> vtkWriter(*gridVariables, partialSol, name);

everything works as expected.

Could be related to the fact that partialSol only stores references to the actual solution sub-vectors. It indeed has a different type than SolutionVector.

C++17 probably would elegantly fix this, since we could just remove SolutionVector from the class template list because it could be inferred from the constructor.