GPU implementation

[mlund]

This thread is for discussing how energy evaluation may be enhanced using GPU's.

Some ideas:

• STL for GPU: https://developer.nvidia.com/thrust
  • see i.e. their "fancy iterators" which can be used to zip up arrays of data
  • also runs on CPUs using OpenMP or TBB: https://github.com/thrust/thrust/wiki/Device-Backends. Use e.g. -DTHRUST_DEVICE_SYSTEM=THRUST_DEVICE_SYSTEM_X where X=CPP, OMP, CUDA, TBB
  • header only
  • thrust is also used in the Aboria particle library, see also Add cell list neighbour search for non-bonded interactions #162
• force biased MC where forces are calculated using GPU code, either internally or via OpenMM