GPU liveness optimization to reduce registers

pystencils/backends/cbackend.py

pystencils/simp/liveness_opts.py

pystencils/simp/liveness_opts_exp.py

pystencils/simp/liveness_permutations.py

pystencils_tests/liveness_opts/compare_seqs.py

pystencils_tests/liveness_opts/count_ops.ipynb

pystencils_tests/liveness_opts/cse_reorder_demo.ipynb

pystencils_tests/liveness_opts/grandchem_test.py

pystencils_tests/liveness_opts/grandchem_test_staggered.py

pystencils_tests/liveness_opts/kernel_split.ipynb

pystencils_tests/liveness_opts/liveness_evo_opt.ipynb

pystencils_tests/liveness_opts/liveness_evo_opt.py

pystencils_tests/liveness_opts/reorder_depth.ipynb

pystencils_tests/liveness_opts/sum_tree.ipynb

...ls_tests/liveness_opts/test_grandchem_staggered_gpu.ipynb

pystencils_tests/liveness_opts/test_liveness_opts.ipynb

pystencils_tests/liveness_opts/test_sched_mu.ipynb

pystencils_tests/liveness_opts/test_steal.ipynb