Refactor constructor so that Input signals and Output signals are connected to...

Refactor constructor so that Input signals and Output signals are connected to ports before traversal is started, that way edge cases of empty SFG's are easily handled

b_asic/signal_flow_graph.py

@@ -403,4 +403,4 @@ class SFG(AbstractOperation):
                 if src.operation.depends(src.index, i):
                     if self._source_depends(port.signals[0].source, input_operation, visited):
                         return True
-        return False
\ No newline at end of file
+        return False