Addition of SYCL implementation of SLAMBench

[agozillon]

This adds a new SYCL module for SLAMBench.

It currently works using ComputeCpp for CPU and GPU and Intels open source SYCL implementation for Intel GPU.

The compilation of the SYCL modules have an external dependency on: https://github.com/agozillon/syclcc which are a set of driver scripts that allow CMake to treat the SYCL compiler as it would treat a normal C++ compiler without the needed addition of any new SYCL specific compile arguments.

Some steps covering this have been added to the README.md. On the note of the README.md I promise the changes aren't as significant as they look from the source <-> source difference! You should be able to just hit the "Display the rich diff" button and it'll show a more realistic view of the changes made.

@pkeir was the main contributor of this patch as you can probably tell by the prolific commit history! So I'm tagging him to help with the discussion of the pull request.