Set workgroup_size=1 for CPUs

Another attempt to resolve #68. Work group size is set to 1 on CPUs.

Incidentally, this also increases performance on CPUs: 5 sec vs 20 sec in [lorenz attractor parameter study](https://github.com/ddemidov/gpgpu_with_modern_cpp/blob/master/src/lorenz_ensemble/vexcl_lorenz_ensemble.cpp) with ensemble of size 1024 on Intel Q9400.