Added direct form IIR tests and updated file for 100% coverage

Added tests for direct form IIR 1 and 2, made small changes to the generators, added multi-tone test for fft and removed unused code for 100 percent test coverage.

Closes: #275 (closed)

test/test_core_operations.py

@@ -413,9 +413,10 @@ class TestSink:
         bfly = Butterfly()
         sfg = bfly.to_sfg()
         s = Sink()
-        sfg1 = sfg.replace_operation(s, "out0")
+        with pytest.warns(UserWarning, match="Output port out0 has been removed"):
+            sfg1 = sfg.replace_operation(s, "out0")
 
-        assert sfg1.output_count == 1
-        assert sfg1.input_count == 2
+            assert sfg1.output_count == 1
+            assert sfg1.input_count == 2
 
-        assert sfg.evaluate_output(1, [0, 1]) == sfg1.evaluate_output(0, [0, 1])
+            assert sfg.evaluate_output(1, [0, 1]) == sfg1.evaluate_output(0, [0, 1])