Adds 'offset_seconds' to PointCloudWithIntensities. (#571)

This is preparing using per-point-unwarping in the `assets_writer` in `cartographer_ros`.

Related to googlecartographer/cartographer_ros#521.

PAIR=@wohe

cartographer/io/points_batch.h

@@ -36,8 +36,8 @@ struct PointsBatch {
     trajectory_id = 0;
   }
 
-  // Time at which this batch has been acquired.
-  common::Time time;
+  // Time at which the first point of this batch has been acquired.
+  common::Time start_time;
 
   // Origin of the data, i.e. the location of the sensor in the world at
   // 'time'.
@@ -50,7 +50,7 @@ struct PointsBatch {
   // Trajectory ID that produced this point.
   int trajectory_id;
 
-  // Geometry of the points in a metric frame.
+  // Geometry of the points in the map frame.
   std::vector<Eigen::Vector3f> points;
 
   // Intensities are optional and may be unspecified. The meaning of these

cartographer/io/xray_points_processor.cc

@@ -216,7 +216,7 @@ void XRayPointsProcessor::Process(std::unique_ptr<PointsBatch> batch) {
     Insert(*batch, &aggregations_[0]);
   } else {
     for (size_t i = 0; i < floors_.size(); ++i) {
-      if (!ContainedIn(batch->time, floors_[i].timespans)) {
+      if (!ContainedIn(batch->start_time, floors_[i].timespans)) {
         continue;
       }
       Insert(*batch, &aggregations_[i]);

cartographer/sensor/point_cloud.h

@@ -32,6 +32,11 @@ typedef std::vector<Eigen::Vector3f> PointCloud;
 struct PointCloudWithIntensities {
   PointCloud points;
   std::vector<float> intensities;
+
+  // For each item in 'points', contains the time delta of when it was acquired
+  // after points[0], i.e. the first entry is always 0.f. If timing
+  // information is not available all entries will be 0.f.
+  std::vector<float> offset_seconds;
 };
 
 // Transforms 'point_cloud' according to 'transform'.