Track the 'start' timestamp to the API to get insight in the timewindow

[toefel18]

Snapshots only contain a timestampTaken.

Suggested improvement: add a 'timestampStarted' so the window of measurements is also clear. This timestamp should be initialized when patan is initialized, and it should be reset when any of the reset() methods is called.

This gives insight in the time window when the measurements were taken.