Cleanup AthExHelloWorld

Merged Tadej Novak requested to merge tadej/athena:cleanup-hello into main

I took the liberty to cleanup AthExHelloWorld a bit:

  • run autopep8 and clang-format so the formatting is at least consistent
  • make separate configuration fragments for tools
  • show how to use flags
  • modernise C++ a bit

I removed start/stop because I'm not even sure what's the purpose.

I'm also thinking if this example should be AthReentrantAlgorithm?

/cc @nstyles @jcatmore @fwinkl @akraszna @ssnyder @tsulaia @zmarshal

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