Define lowering and adjacent blocks merging as transformations

src/pairs/ir/block.py

src/pairs/ir/mutator.py

src/pairs/sim/arrays.py

src/pairs/sim/cell_lists.py

src/pairs/sim/lattice.py

src/pairs/sim/lowerable.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

src/pairs/sim/vtk.py

src/pairs/transformations/lower.py

src/pairs/transformations/merge_adjacent_blocks.py

src/pairs/transformations/prioritize_scalar_ops.py