New cpu cumulative sensors

In this second merge request I implemented a few cumulative sensor for the cpu: the max/min/average value of the clock frequencies and temperatures of the various cores/cpus. The reason is behind is quite simple: modern cpus have more and more cores and all of them have their sensors; with such large number of sensors, a summary of those is needed if you want to know the state of your processor with just a glance.

All of these new sensor should work also on bsd, but I've been able to test them only on linux.