#2 Timestamp in mSensorMonitor

[Daniel Germann]

An automatically generated timestamp (in the first
column of the output file) would be convenient.

An enum in the constructor of
mSensorMonitor/mSwitchableSensorMonitor could
choose between
- eTS_NONE (default)
- eTS_LOOPTIME (time passed since first_sense)
- eTS_ABSTIME (absolute time from tTime)
or whatever.