Resolve "Signal Interface"

b_asic/basic_operation.py

b_asic/core_operations.py

b_asic/port.py

b_asic/signal.py

b_asic/signal_flow_graph.py

test/fixtures/__init__.py

test/fixtures/signal.py

test/port/__init__.py

test/port/test_inputport.py

test/port/test_outputport.py

test/port/test_port.py

test/traverse/test_traverse_tree.py