Add decorators for kernel blocks

examples/kernels

examples/kernels.pdf

examples/lj_ns.cpp

src/pairs/ir/block.py

src/pairs/sim/arrays.py

src/pairs/sim/cell_lists.py

src/pairs/sim/lattice.py

src/pairs/sim/neighbor_lists.py

src/pairs/sim/pbc.py

src/pairs/sim/properties.py

src/pairs/sim/read_from_file.py

src/pairs/sim/simulation.py

src/pairs/sim/variables.py