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

test/test_sfg.py

@@ -270,6 +270,7 @@ class TestReplaceOperations:
 
         assert 'mad1' in _sfg._components_by_id.keys()
         assert {add1, mul1} not in _sfg.operations
+        assert _sfg.input(0).signals
 
     def test_replace_neg_add_with_sub(self):
         in1 = Input()
@@ -284,6 +285,7 @@ class TestReplaceOperations:
 
         assert 'sub1' in _sfg._components_by_id.keys()
         assert {add1, neg1} not in _sfg.operations
+        assert _sfg.input(0).signals
 
     def test_different_input_output_count(self, operation_tree):
         sfg = SFG(outputs=[Output(operation_tree)])