[WIP] clang-format

[ax3l]

Draft of a .clang-format rule for our projects, based on clang-format 6.0+.

CC @psychocoderHPC, @C0nsultant, @BenjaminW3 https://github.com/ComputationalRadiationPhysics/alpaka/issues/419

Most Important Limitations

• AlignAfterOpenBracket/BinPack*: multi-parameter breaks for N>1 (template, function param, init lists, etc.) do not support 1-element-per-line; closing bracket on newline missing (H)
• NamespaceIndentation: Inner is not exactly what we do (+)
• double-newline after include blocks is not enforced, but MaxEmptyLinesToKeep: 2 keeps it
• can not enforce empty newlines between function/class, etc. blocks: SO
• IndentPPDirectives: Hash: works but does not count the hash as space (+)
• desc.add_options()()()... syntax in boost::program_options looks weird - but we should switch to, e.g. CL11 anyway: https://github.com/ComputationalRadiationPhysics/picongpu/issues/2421

(H) probably some work to add & upstream but quite central in our style (+) easy to add, maybe something to upstream?

Misc Notes

• the IncludeCategories: block is project specific, example is shown for PIConGPU

• online tool: https://zed0.co.uk/clang-format-configurator/ (currently not up-to-date with clang 6.0: https://github.com/zed0/clang-format-configurator/issues/15)

• references: https://clang.llvm.org/docs/ClangFormat.html https://clang.llvm.org/docs/ClangFormatStyleOptions.html

[j-stephan]

I think we can close this once #41 is merged.

[bernhardmgruber]

@psychocoderHPC can you please close this one please?

[psychocoderHPC]

clang format file is now final with #41