Add ignition version of nav_msgs/OccupancyGrid (#138)

* Add ignition version of nav_msgs/OccupancyGrid

Signed-off-by: Michael Carroll <michael@openrobotics.org>

* Add map_load_time

Signed-off-by: Michael Carroll <michael@openrobotics.org>

proto/ignition/msgs/occupancy_grid.proto

@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2021 Open Source Robotics Foundation
+ *
+ * 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.
+ *
+*/
+
+syntax = "proto3";
+package ignition.msgs;
+option java_package = "com.ignition.msgs";
+option java_outer_classname = "OccpuancyGridProtos";
+
+/// \ingroup ignition.msgs
+/// \interface OccupancyGrid
+/// \brief This message is designed to mimic ROS nav_msgs/OccupancyGrid to
+/// facilitate transfer/conversion of data between Ignition and ROS.
+///
+/// See: http://docs.ros.org/melodic/api/nav_msgs/html/msg/OccupancyGrid.html
+
+import "ignition/msgs/header.proto";
+import "ignition/msgs/pose.proto";
+import "ignition/msgs/time.proto";
+
+
+message OccupancyGrid
+{
+  message MapMetaInfo
+  {
+    /// \brief The map load time
+    Time map_load_time = 1;
+
+    /// \brief The map resolution (meters/cell)
+    double resolution = 2;
+
+    /// \brief The map width (cells)
+    uint32 width = 3;
+
+    /// \brief The map height (cells)
+    uint32 height = 4;
+
+    /// \brief  The origin of the map.  This is the real-world pose of the
+    /// cell (0,0) in the map
+    Pose origin = 5;
+  };
+
+  /// \brief Optional header data. This should contain time of map validity
+  /// and the coordinate frame ID.
+  Header header = 1;
+
+  /// \brief Metadata for the map.
+  MapMetaInfo info = 2;
+
+  /// \brief The map data, in row-major order, starting with (0,0).
+  /// Occupancy probabilities are in the range [0,100].  Unknown is -1.
+  bytes data = 3;
+};