Minor sparse_pose_graph.proto improvement (#226)

cartographer/mapping/proto/sparse_pose_graph.proto

@@ -44,8 +44,9 @@ message SparsePoseGraph {
     // Pose of the scan relative to submap, i.e. taking data from the scan frame
     // into the submap frame.
     optional transform.proto.Rigid3d relative_pose = 3;
-    // 6x6 square root inverse of the covariance matrix.
-    repeated double sqrt_Lambda = 4;  // NOLINT
+    // 6x6 square root inverse of the covariance matrix. This is stored in
+    // column-major order for ease of use with Eigen::Map.
+    repeated double sqrt_Lambda = 4 [packed = true];  // NOLINT
     optional Tag tag = 5;
   }