log, warning and error messages in workflow mode

In workflow mode the interpreter and fireworks write log messages. These should not be confused with errors and warnings that are written on stderr. In Jupyter the logs are shown in a separate pane. In the CLI these can be suppressed by setting the log level to WARNING or ERROR (then INFO and DEBUG messages are suppressed). We have to figure out how to do this in the most elegant way. We can look at fireworks how they manage this. In all cases, in workflow mode we have another setting: log level.