title: list memory usage in ipython and jupyter
body: <p>I have a few (almost ten) Gb of memory taken by the ipython kernel. I think this is coming from large objects (matrices, lists, numpy arrays, ...) that I might have produced during some operation and now I do not need anymore.</p>
  
  <p>I would like to <strong>list all of the objects I have defined</strong> and <strong>sort them</strong> by their memory footprint. Is there a simple way to do that? For certain types there is <code>nbytes</code> method, but not for all ... so I am looking for a general way to list all objects I have made and their memory occupation.</p>