Beta-RC5 Release

# Do NOT merge this branch yet!

This will be the next release of PIConGPU.
We will wait for at least *Monday* to check for obvious bugs before it goes in.

Thanks to all developers that contributed to this new release. We made major improvements in our **plugin and restart system** (thanks to @f-schmitt-zih). @psychocoderHPC refactored a huge amount of **particle** and **compile-time** related routines. We got cool new features related to **xdmf** and tools like **ParaView**, can read in new gas profiles from **png** images and exposed the **base seed** in `seed.param`. There have been some new **window functions** added to the radiation plugin to avoid ringing effects on sharp boundaries/periodic systems. A new plugin can now compute 2D **phase space** distributions on the fly.

We **fixed** a good amount of bugs and started writing a **comprehensive wiki** for our users.


## To Do

- [ ] *wait* for further bugs to show up
- [x] @psychocoderHPC one of your pulls improved the computeCurrent routine quite a lot. Can you remember which one or did I mismatch that with the coming scheme?
- [x] update the release date -> **Release day will be Wednesday, 28th of May**