Fix code generation for LJ

examples/lj.py

src/pairs/code_gen/cgen.py

src/pairs/ir/atomic.py

src/pairs/ir/bin_op.py

src/pairs/ir/properties.py

src/pairs/sim/contact_history.py

src/pairs/sim/properties.py

src/pairs/sim/simulation.py

src/pairs/transformations/modules.py