collect CUDA includes and calls, separate from CPU-only mode, leave out

- collect CUDA includes in device_alternate.hpp
- add guards for CUDA code
- move GPU code into cu from cpp
- make CUDA includes and libraries conditional in Makefile
- drop CUDA dependency from travis-ci build
switch travis build to CPU-only flag, drop runtestnogpu target
relax benchmark test timing for cheap hardware / CI build