0.6.0: New Backends and Useability Improvements

This release adds two new backends: OpenMP 5 target offload and OpenACC.

We improved useability of alpaka API by flattening the namespace hierarchies and making some renamings.
A full list of API changes is avaiable in the changelog.
New features include support for warp voting functions, setting schedule for OpenMP2Blocks backend and simplified interfaces for atomic functions.

For cmake-based builds, we no longer automatically enable all available backends.
Now a user has to explicitly enable the backends to be used.

The readthedocs documentation was extended with a cheatsheet and support for Doxygen.

This release is adding compatibility to the latest CUDA releases up to 11.2.
The HIP backend is now more stable and supports HIP 3.5+.
We recommend using the latest HIP version to benefit from its fast improvements.

We fixed many bugs and improved support for Intel C++ compiler.