Add GPU support

[psyhtest]

So far, we have successfully run the OpenVINO program on the CPU. Supporting the GPU would be a logical next step.

We foresee several challenges:

• The CPU code and the GPU code are deceptively similar but do have important differences. The GPU code should be more general.

• The OpenVINO library should be built differently for the GPU than for the CPU. In particular, the ENABLE_CLDNN flag should be ON. The GNA, the VPU and the DLIA (FPGA?) plugins should be OFF.

• However, we should not enable it by default, because this can lead to weird issues. We should probably introduce flags USE_GPU and USE_CPU similar to ArmNN's USE_OPENCL and USE_NEON.

• CMakeLists.txt for the GPU is not up-to-date with CMakeLists.txt for the CPU.