Added missing file.
Added access to gpu forces to SystemInterface and implementation.
Make Interface enable particle comm if needed.
Added simple test force to illustrate interface usage.
Merge branch 'master' of github.com:espressomd/espresso
Merge branch 'master' of github.com:espressomd/espresso
Removed stray debug messages.
HarmonicForce: Added Cuda wrappers.
Merge branch 'master' of github.com:espressomd/espresso
Cuda communication cleanup., small fix to HarmonicForce.
HarmonicForce: Doc and NEWS.