Add ballast app, blockforest optimizations, I/O, Vector3 operators

CMakeLists.txt

examples/modular/ballast/ballast.cpp

examples/modular/ballast/ballast.py

examples/modular/ballast/boxes.txt

examples/modular/ballast/spheres_bed_with_box.txt

runtime/math/Vector3.hpp

runtime/utility/read_from_file.cpp

runtime/utility/read_from_file.hpp

src/pairs/code_gen/accessor.py

src/pairs/sim/domain_partitioning.py