Fix diagnostics appearance

[toliver]

To make the rqt plugin rqt_robot_monitor show the fingers diagnostics, we removed the nested categories (see e059203).

This makes the diagnostics for the fingers appear at the same level as other tags. Somehow this is no very clean, as they are alphabetically ordered and the tactiles appear between the Ring Finger and the Thumb.

We should find a way to make the displayed info look better.

To test this you will probably need a hand or at least the finger rig (on Gavin's shelf).

Some explanation:

• the rqt_robot_monitor reads and displays the info from the diagnostics_agg topic
• the diagnostics_agg topic is published by the diagnostics_aggregator node. It reads the diagnostics topic and aggregates the information considering the parameters in the diagnostic_analyzer.yaml file.

I don't know if the way to fix this will involve only tweaking this diagnostic_analyzer.yaml in sr_config, or will involve modifications of rqt_robot_monitor in a fork of https://github.com/ros-visualization/rqt_robot_plugins