[SIGMON-423] plots fixed in the QH notebooks

issue: https://its.cern.ch/jira/browse/SIGMON-423

Both inline and notebook backends implicitly display all plots that are created in a cell. To prevent this behaviour we used to use agg backend.
It turned out that it's not possible to switch on agg more than once during a kernel lifetime without workarounds.
I decided to use %%capture (https://notebook.community/lifeinoppo/littlefishlet-scode/RES/REF/python_sourcecode/ipython-master/examples/IPython%20Kernel/Capturing%20Output) in the cell that creates figures instead.