Don't recalculate timing offsets in inner loop

Moving the timing offset calculation up one loop has no effect on the
results, since it depends on the packet, but not any specific block.

spherical_point_cloud.go

@@ -24,10 +24,10 @@ type SphericalPoint struct {
 func (s *SphericalPointCloud) UnmarshalPacket(packet *Packet) {
 	// duration is in nanoseconds and Velodyne timestamp in microseconds
 	s.Timestamp = time.Duration(packet.Timestamp) * time.Microsecond
+	timingOffsets := calculateTimingOffset(packet.ReturnMode)
 	for i := range packet.Blocks {
 		block := &packet.Blocks[i]
 		azimuth := block.Azimuth
-		timingOffsets := calculateTimingOffset(packet.ReturnMode)
 		for j := range block.Channels {
 			channel := &block.Channels[j]
 			if j == 16 {