Request for ability to hide output from notebooks.

Currently in the Python implementation of Notebooks, we have output that are hidden.

• Today these are neither displayed in our old Python Notebooks nor in Jupyter
• As far as I can tell, most of these outputs end up as Rich Displays, containing an object with a single key. However no real data for display purposes.
• The outputs are not displayed, however they are persisted on disck (i.e. they are part of the ipynb content).

Adding an issue for what was discussed (forgot to add this issue last week).
@rebornix /cc