Stabilization: pnetwriter

Open tasks:

• Add example to test restart and reading from (@pgupta suggested small mesh, e.g. 4x4x4 mesh).
• Check "manually" if restarting and reading still works in MPI-parallel.
• Rebase branch on master
• Can we add an automatic test to CI that checks this?
• Check if restarting relaxation works (it starts from where it ended)