Warn if CPU scaling governor is not set to performance

[pyk]

Frequency scaling introduces significant noise in benchmarks. Most Linux distros default to "powersave" or "schedutil".

It would be great if the tool checked /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor on startup.

If it is not set to "performance", we should print a warning to the user so they know their environment might be unstable.