get_random_n_shapes in DiagramGenerator.py

[guillesanbri]

This method is fixed on 5 as the current default values to generate the size of a shape (max) is
min_size_diagram2(1+1.5)/n_shapes
and therefore if the n_shapes is less than 5 the shape can be bigger than the diagram, raising an error in the random integer method. However, if this default values change this method will break.

TODO: Link this method to the size formula in SyntheticDiagram.