Optimize BlockForest: skip empty AABBs, crop subdom, gen halo cells

examples/benchmarks/sd_static.cpp

examples/benchmarks/sd_static_triangular.cpp

examples/benchmarks/spring_dashpot.py

examples/benchmarks/spring_dashpot_no_pbc.py

examples/modular/force_reduction.cpp

examples/modular/sd_1.cpp

examples/modular/sd_2.cpp

examples/modular/sd_3_CPU.cpp

examples/modular/sd_3_GPU.cu

examples/modular/sd_4.cpp

examples/modular/sphere_box_global.cpp

runtime/domain/ParticleDataHandling.hpp

runtime/domain/block_forest.cpp

runtime/domain/block_forest.hpp

runtime/domain/boundary_weights.cpp

runtime/domain/boundary_weights.cu

runtime/domain/boundary_weights.hpp

runtime/domain/domain_partitioning.hpp

runtime/domain/regular_6d_stencil.cpp

runtime/domain/regular_6d_stencil.hpp