Fix some issues

examples/dem.py

src/pairs/code_gen/cgen.py

src/pairs/ir/mutator.py

src/pairs/ir/properties.py

src/pairs/ir/select.py

src/pairs/mapping/funcs.py

src/pairs/sim/contact_history.py

src/pairs/sim/interaction.py

src/pairs/transformations/__init__.py

src/pairs/transformations/expressions.py