Add a ROS message for landmark observations. (#732)

[Landmark RFC](https://github.com/googlecartographer/rfcs/blob/master/text/0011-landmarks.md)

cartographer_ros_msgs/CMakeLists.txt

@@ -24,6 +24,8 @@ find_package(catkin REQUIRED COMPONENTS message_generation ${PACKAGE_DEPENDENCIE
 
 add_message_files(
   FILES
+    LandmarkEntry.msg
+    LandmarkList.msg
     StatusCode.msg
     StatusResponse.msg
     SubmapList.msg

cartographer_ros_msgs/msg/LandmarkEntry.msg

@@ -0,0 +1,18 @@
+# 2018 The Cartographer Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+string id
+geometry_msgs/Pose tracking_from_landmark_transform
+float64 translation_weight
+float64 rotation_weight

cartographer_ros_msgs/msg/LandmarkList.msg

@@ -0,0 +1,16 @@
+# Copyright 2018 The Cartographer Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+std_msgs/Header header
+LandmarkEntry[] landmark