Upgrade to FLAME GPU 1.5

[ptheywood]

When FLAME GPU 1.5 is released, we should update this example and the associated documentation / instructions.

We should use tags to maintain easy to access tutorials for each version of FLAME GPU.

[ptheywood]

This is required to build the tutorial with GCC >= 6 / CUDA >= 9 on linux.

Ideally we should also switch to host initialisation of agents, to remove the need for XML generation.

I would also suggest moving to RST / MD for the instructions, so they do not have to be hosted on gdrive, and could be directly coupled with the tutorial rather than in a separate repo.