Implemented hook for measuring GPU times

[brownbaerchen]

I split off the timings from the default hook and implemented a version for GPU timings. This works with CUDA events, see the CuPy documentation for details on how to measure GPU timings.

Note that the time elapsed on CPU is not in general correlated with the time to run an operation on GPU. There are some operations that implicitly synchronise device and host, such as memory transfers between the two, resulting in similar CPU and GPU timings in pySDC. I did not find a meaningful difference in average timing_step between the two hooks in my experiments. Still, I think we need a hook that makes this reliable by explicitly synchronising.