Add SyncModes, Shape definitions, experimental MPI NBC

examples/benchmarks/spring_dashpot.py

examples/benchmarks/spring_dashpot_no_pbc.py

examples/modular/ballast/ballast.py

examples/modular/force_reduction.cpp

examples/modular/force_reduction.py

examples/modular/linear_spring_dashpot.py

examples/modular/lsd_1.cpp

examples/modular/sphere_box_global.py

examples/modular/spring_dashpot.py

runtime/pairs.cpp

runtime/pairs.hpp

runtime/pairs_common.hpp

src/pairs/__init__.py

src/pairs/code_gen/interface.py

src/pairs/ir/features.py

src/pairs/ir/properties.py

src/pairs/ir/sync_modes.py

src/pairs/mapping/funcs.py

src/pairs/sim/cell_lists.py

src/pairs/sim/comm.py