Resolve "System tests iteration 1"

Closes #62 (closed)

Added test cases for creating and simulating the simple filter used in the first lab for TSTE87. The SFG used in the labs gives output from the SFG as an array, although, our program seems to give these outputs values incrementally. I expect that we will need to change this in the future to be able to plot the output values.

Also refactorized some test cases.

test/test_sfg.py

@@ -45,7 +45,7 @@ class TestInit:
         assert sfg.input_count == 0
         assert sfg.output_count == 1
 
-class TestStr:
+class TestPrintSfg:
     def test_one_addition(self):
         inp1 = Input("INP1")
         inp2 = Input("INP2")
@@ -200,7 +200,6 @@ class TestComponents:
 
 
 class TestReplaceComponents:
-
     def test_replace_addition_by_id(self, operation_tree):
         sfg = SFG(outputs=[Output(operation_tree)])
         component_id = "add1"