Add empirical maximum FLOP rate measurement

[nchristensen]

No description provided.

[nchristensen]

https://github.com/krrishnarraj/clpeak seems to work for the Titan V and its gives global memory bandwidth numbers close to mine.

[nchristensen]

It gives very different and sometimes wildly varying latency numbers though. clpeak probably measures the kernel launch and kernel completion latency whereas the benchmarks here measure the device memory latency.