Improve timestamp granularity of different measurements within a scan

[tobiasneumann]

The current message for a RadarScan just stores one time stamp for all returns. And this time stamp is not clearly defined.

Proposed change

• Add the field stamp_offset to RadarReturn, which will be used relatively to the time stamp of the time stamp in the field header of RadarScan.
• Define the time stamp of the field header of RadarScan to be of the first return.