Adjust code generation for feature properties

examples/lj.py

examples/lj_onetype.py

src/pairs/code_gen/cgen.py

src/pairs/ir/features.py

src/pairs/ir/particle_attributes.py

src/pairs/ir/properties.py

src/pairs/sim/features.py

src/pairs/sim/read_from_file.py

src/pairs/sim/simulation.py