Add RFC for delete / load interface in ROS. (#41)

Signed-off-by: Michael Grupp <[email protected]>

text/0029-delete-load-in-cartographer-ros.md

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+# Delete / load interface in Cartographer ROS
+
+## Summary
+[summary]: #summary
+
+Expose the delete / load interface of the pose graph in Cartographer ROS.
+
+## Motivation
+[motivation]: #motivation
+
+The pose graph / map builder interface supports deleting trajectories and loading states at runtime.
+The motivation of RFC-23 for why delete/load is useful also applies here.
+
+So far, we have gRPC services for doing that:
+* [DeleteTrajectory](https://github.com/googlecartographer/cartographer/blob/master/cartographer/cloud/proto/map_builder_service.proto#L111)
+* [LoadStateFromFile](https://github.com/googlecartographer/cartographer/blob/897762675caffdb8f21d07ff01824528b56c2e8b/cartographer/cloud/proto/map_builder_service.proto#L161)
+
+
+The goal of this RFC is to make this accesible from Cartographer ROS.
+
+
+## Approach
+[approach]: #approach
+
+### New ROS services
+
+`/delete_trajectory`:
+```
+int32 trajectory_id
+---
+cartographer_ros_msgs/StatusResponse status
+```
+
+`/load_state_from_file`:
+```
+string file_path
+bool load_frozen_state
+---
+cartographer_ros_msgs/StatusResponse status
+```
+
+### Handlers
+
+`HandleDeleteTrajectory` needs to:
+* check the requested trajectory ID exists and is in trajectory state `FINISHED` or `FROZEN` (otherwise [the deletion work item crashes](https://github.com/googlecartographer/cartographer/blob/master/cartographer/mapping/internal/2d/pose_graph_2d.cc#L613)) - [CheckTrajectoryState](https://github.com/googlecartographer/cartographer_ros/pull/1262) will be useful for that
+* call `DeleteTrajectory`
+
+`HandleLoadStateFromFile` just calls `LoadStateFromFile`.
+
+## Discussion Points
+[discussion]: #discussion
+
+### Potential race conditions, necessary checks
+
+* deletion is handled by a trimmer, which means it is delayed until the next optimization
+* deletion of frozen trajectories requires a patch, see: https://github.com/cartographer-project/cartographer/pull/1767