Create benchmarks/ dir and move *_driver from examples

Arndt, Danielrequested to merge
github/fork/aprokop/create_benchmark_dir into master

Created by: aprokop

ArborX configuration with

  -D ARBORX_ENABLE_TESTS=OFF
  -D ARBORX_ENABLE_EXAMPLES=ON
  -D ARBORX_ENABLE_BENCHMARKS=OFF

no longer requires boost or google benchmark.

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