gbench

- It also adds Google's benchmarch compare.py script
  - It is installed to the build directory.

- It add a wrapper script called compare-benchmarks.py which:
  - Let you run each of the benchmarks with different devices

- It adds a README.md explaining how to run the benchmarks

- BenchmarkDeviceAdapter input size range parametrized at compile time

Signed-off-by: Vicente Adolfo Bolea Sanchez <vicente.bolea@kitware.com>