Merge branch 'feature/openmp_opencl_target_fixstars' into merge/openmp_opencl_target_fixstars

Conflicts:
	runtime/buffer.cc
	runtime/buffer_cuda.cu
	runtime/grid_mpi.cc
	runtime/grid_util.cc
	tests/system_tests/run_system_tests.sh.cmake
	translator/physisc.cc
	translator/reference_translator.cc

The four original targets pass the system tests.
Merge branch 'merge/openmp_opencl_target_fixstars' of github.com:naoyam/physis into merge/openmp_opencl_target_fixstars
Merge branch 'merge/openmp_opencl_target_fixstars' of github.com:naoyam/physis into merge/openmp_opencl_target_fixstars
Merge branch 'merge/openmp_opencl_target_fixstars' of github.com:naoyam/physis into merge/openmp_opencl_target_fixstars
Emphasize the OpenCL and OpenMP targets are experimental.
Merge branch 'merge/openmp_opencl_target_fixstars' into develop