Fix support for Keplar (K20 & K80) GPUs. (#470)

Fixes issue #467 

* Add GPU synchronization points where required for Kepler to coordinate CPU access of managed memory.
* Use hand-rolled double precision atomic addition for Kelper targets.
* Replace `ARB_WITH_CUDA` build option with `ARB_GPU_MODEL` option that takes one of 'none', 'K20', 'K80' or 'P100', and set up source-code defines accoringly.
* Clean up of redundant compiler flags and defines no longer required now that the project uses separate compilation for CUDA sources.