More code quality fixes

b_asic/GUI/simulate_sfg_window.py

b_asic/GUI/utils.py

b_asic/operation.py

b_asic/signal_flow_graph.py

test/conftest.py

test/fixtures/operation_tree.py

test/fixtures/signal_flow_graph.py

test/test_fast_simulation.py

test/test_sfg.py

test/test_signal.py

test/test_simulation.py