Expand mesh diff to accept lists or array of meshes as input

...static/examples/meshlib/plot_calculate_diff_thumbnail.png

docs/examples/howto_meshlib/plot_calculate_diff.py

docs/examples/howto_meshplotlib/plot_special.py

docs/examples/howto_meshplotlib/plot_with_custom_fig_ax.py

...ples/howto_meshplotlib/plot_xy_labels_with_shared_axes.py

ogstools/meshlib/__init__.py

ogstools/meshlib/data_processing.py

ogstools/studies/templates/convergence_study.py

tests/test_meshlib.py

tests/test_meshplotlib.py