Resolve "Operation Replacement in a SFG"

Added method with tests for replacing multiple operations in a sfg with another operation with the same amount of input / output ports. Closes #17

b_asic/signal_flow_graph.py

@@ -444,7 +444,7 @@ class SFG(AbstractOperation):
                 _signal.remove_source()
                 _signal.set_source(operation.output(index_out))
        
-        return _sfg
+        return _sfg()
 
     def _evaluate_source(self, src: OutputPort, results: MutableResultMap, registers: MutableRegisterMap, prefix: str) -> Number:
         src_prefix = prefix