Example GPU app

[David P. Anderson]

Here's an app (diff eq solver) with variants for several GPU frameworks
(CUDA, OpenCL, Metal) as well as OpenMP (multicore CPU).
https://github.com/ProteusMRIgHIFU/BabelViscoFDTD

This might be useful for experimenting with these frameworks.
I hadn't realized that Metal could be used for non-graphics apps.

-- D