Close #557 Beta-RC6 Release

This is the `beta-rc6` release of PIConGPU.

@ComputationalRadiationPhysics/picongpu-maintainers please review the [Changelog](https://github.com/ComputationalRadiationPhysics/picongpu/compare/5e1eee501dafdc3202334707e0ff9a776195df63...release-beta-rc6?name=release-beta-rc6&short_path=4ac32a7#diff-4ac32a78649ca5bdd8e0ba38b7006a1e) for typos or topics you might miss.

While the `param` structure for species is still rather *messy*, initial **multiple/flexible species** support was added and is yet still limited to *two species*.
An experimental `mallocMC` powered [N-species branch](https://github.com/ax3l/picongpu/commits/example-multilayer) exists and the cleanup and in-cooperation will come with the next release.

The **checkpoint system** was refactored and unified, also incooperating extreme high file I/O bandwidth with **ADIOS 1.7+** support. The **JetsonTK1** development kit (32bit ARM host side) is now experimentally supported by `libPMacc` and `PIConGPU`. The **ZigZag current deposition scheme** was implemented providing **40% to 50% speedup** over our optimized Esirkepov implementation.

### Misc

Benjamin Worpitz was added to the developers list. Again, welcome :sparkles: and thanks for your first pull requests!
I replaced the 3D eye catcher in the [README.md](https://github.com/ComputationalRadiationPhysics/picongpu/commit/93be573d02dae0833706097bc0f44839bd0cb963) with a 3D rendered one from the in-situ vis.

### To Do

 - [x] wait for comments
 - [x] @f-schmitt-zih shall I push the PIConGPU/png2gas libSplash dependency to 1.2.3+ ? Both are 1.2.0 right now. At least 1.2.2+ would be reasonable so they write fixed-size data fields.
 - [x] merge at: Tuesday, 25th of Nov