Resolve "Operation to SFG Conversion"

Added to_sfg method, MAD operation and some tests.

Closes #80 (closed)

b_asic/signal_flow_graph.py

@@ -284,7 +284,7 @@ class SFG(AbstractOperation):
     def split(self) -> Iterable[Operation]:
         return self.operations
     
-    def to_sfg(self) -> SFG:
+    def to_sfg(self) -> 'SFG':
         return self
 
     def inputs_required_for_output(self, output_index: int) -> Iterable[int]: