polishing round 2

docs/conf.py

docs/examples/howto_meshplotlib/plot_solid_mechanics.py

docs/examples/howto_propertylib/plot_propertylib.py

...les/howto_studies/plot_convergence_study_nuclear_decay.py

ogstools/meshplotlib/core.py

ogstools/meshplotlib/plot_features.py

ogstools/propertylib/mesh_dependent.py

ogstools/propertylib/presets.py

ogstools/propertylib/property.py

ogstools/studies/convergence/convergence.py

tests/test_mechanics.py

tests/test_propertylib.py