Improve icon-vis and psyplot exercises

[AnnikaLau]

• The psyplot exercise should be moved to Part3_icon-vis because the users should be introduced to ICON before the exercise or be able to skip the exercise, in case they don't work with unstructured data.
• The update function of psyplot requires the automatic background closing of figures to be disabled. However, this can lead to figures being plotted below or above each other when updating a figure. The users should be informed that they manually have to close the figure with plot_icon.close() or by closing all open figures with plt.close('all').

[AnnikaLau]

• Moved psyplot exercise to Part 3: f947b002b488ad459012309a562a1d50a6f74a62
• Added information about closing figures in psyplot exercise: 596c875383699839022375c1ccdd1d1320af49ba