Beta-RC3 Release

This is the round up for the next pre-release of PIConGPU, **beta-rc3**.

Please refer to the [change log](https://github.com/ax3l/picongpu/blob/release-beta-rc3/CHANGELOG.md#open-beta-rc3) for details.

**Release highlights:**
- parallel hdf5 output with libSplash 1.1
- ADIOS output support
- a good amount of bug fixes in both PIConGPU and libPMacc
- mapped memory support