Cleanup AthExHelloWorld

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