[stokesdarcy2] Port VTK output functions to new infrastructure
[VTK] Add an initial version of a VTK function base class
Add new Poisson example with Dirichlet-Neumann iteration
Some cleanup in dirichlet-neumann example

Most importantly, the actual problem setup (after grid creation and
space definition) is now moved to a separate function.
[SubDomainSet] Add second constructor that takes an initializer list

That's a whole lot easier than the weird variable template argument
trick, and since we can now rely on compilers supporting this feature,
let's profit from it.
[Tests] Clean up Dirichlet-Neumann example
[Tests] Add new test for measuring assembly performance