Suggestion: add "required" topics for -no-ui

[JanQuenzel]

When running with the no-ui flag, it would be nice to get an error message if some required message like sensor data is not recorded.

[xqms]

Interesting idea! I wonder, however, what the criterion for recorded/not recorded could be. I think it's not that easy to determine, since some messages are generated quite infrequently.

You can also take a look at the rqt GUI we are developing in the feature/gui branch, maybe it addresses your use case already.

[JanQuenzel]

Required topics might be added with a certain allowable timeout or minimal frequency. e.g.: --required-topics /os_cloud_node/points:5hz /os_cloud_node/imu:0.1s with the colon as a special character to discern topic from frequency / timeout