Deadtime

This solves issue #59 : now all CPM-related output consistently uses deadtime-compensated CPM values. Serial interface now only supports one GETCPM (or {"get":"cpm"} ) command, which returns all values, rather than several separate commands. This way, there is no ambiguity anymore.

One open point for discussion if that the logger uses uncompensated values for CPM30. The difference between compensated and uncompensated is very small, though, so this might or might not be an issue. To be further discussed.