Partition in XY for DEM case

examples/dem.py

examples/lj.py

runtime/domain/regular_6d_stencil.cpp

runtime/domain/regular_6d_stencil.hpp

runtime/pairs.cpp

runtime/pairs.hpp

runtime/pairs_common.hpp

src/pairs/__init__.py

src/pairs/code_gen/cgen.py

src/pairs/sim/domain_partitioners.py

src/pairs/sim/simulation.py