Resolve "Operation to SFG Conversion"

Added to_sfg method, MAD operation and some tests.

Closes #80 (closed)

b_asic/signal_flow_graph.py

@@ -283,6 +283,9 @@ class SFG(AbstractOperation):
 
     def split(self) -> Iterable[Operation]:
         return self.operations
+    
+    def to_sfg(self) -> SFG:
+        return self
 
     def inputs_required_for_output(self, output_index: int) -> Iterable[int]:
         if output_index < 0 or output_index >= self.output_count: