[all] uniform typehint of UnstructuredGrid

docs/examples/howto_meshplotlib/plot_meshplotlib_2d.py

docs/examples/howto_meshplotlib/plot_meshplotlib_3d.py

docs/examples/howto_meshplotlib/plot_with_custom_fig_ax.py

...ples/howto_meshplotlib/plot_xy_labels_with_shared_axes.py

ogstools/meshlib/boundary_subset.py

ogstools/meshlib/region/region.py

ogstools/meshplotlib/core.py

ogstools/propertylib/matrix.py

ogstools/propertylib/presets.py

ogstools/propertylib/property.py

ogstools/propertylib/vector.py

ogstools/studies/convergence/convergence.py

...ls/studies/convergence/examples/steady_state_diffusion.py

ogstools/studies/templates/convergence_study.py