OpenMP handling

[MaxThevenet]

Doc to use OpenMP efficiently on Juwels, and make openmp default

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[SeverinDiederichs]

Hm, could we add a statement that it is absolutely forbidden to run production runs on the JUWELS Booster on CPU?
Maybe we should have a general CPU section in the documentation

[MaxThevenet]

Right, I just added a warning. The booster is the only place were this was really tested, unfortunately, so I don't want to make it more general before we test it somewhere else.

[SeverinDiederichs]

Thanks!