[examples] Add an example with a pure Neumann problem

This is a small modification of the `poisson-pq2` example
where the boundary treatment has been removed. It's added
here to demonstrate how to conveniently add the kernel projection
to the iterative solver. Theoretically PCG is invariant under
contributions in the kernel, such that it is sufficient to
project the final solution. However, due to round-off errors
and the ill-conditioning of the matrix, these contribution
can be amplified on fine grids which severely slows down
the solver (e.g. about twice as many steps).