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.