rqt_robot_monitor displays diagnostics_agg topics messages that are published by
diagnostic_aggregator. rqt_robot_monitor is a direct port to rqt of
robot_monitor. All diagnostics are fall into one of three tree panes depending
on the status of diagnostics (normal, warning, error/stale). Status are shown in
trees to represent their hierarchy. Worse status dominates the higher level
status. Ex. 'Computer' category has 3 sub devices. 2 are green but 1 is error.
Then 'Computer' becomes error. You can look at the detail of each status by
double-clicking the tree nodes. Currently re-usable API to other pkgs are not
explicitly provided.