[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 for moderately refined grids here).