Add first code to support DEM

Makefile

examples/dem.py

examples/lj.py

src/pairs/code_gen/cgen.py

src/pairs/ir/bin_op.py

src/pairs/ir/const_vector.py

src/pairs/ir/loops.py

src/pairs/ir/math.py

src/pairs/ir/operators.py

src/pairs/ir/properties.py

src/pairs/ir/select.py

src/pairs/ir/symbols.py

src/pairs/mapping/funcs.py

src/pairs/sim/contact_history.py

src/pairs/sim/neighbor_lists.py

src/pairs/sim/properties.py

src/pairs/sim/simulation.py

src/pairs/transformations/expressions.py