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

b_asic/core_operations.py

@@ -229,3 +229,18 @@ class Butterfly(AbstractOperation):
 
     def evaluate(self, a, b):
         return a + b, a - b
+
+class MAD(AbstractOperation):
+    """Multiply-and-add operation.
+    TODO: More info.
+    """
+
+    def __init__(self, src0: Optional[SignalSourceProvider] = None, src1: Optional[SignalSourceProvider] = None, src2: Optional[SignalSourceProvider] = None, name: Name = ""):
+        super().__init__(input_count = 3, output_count = 1, name = name, input_sources = [src0, src1, src2])
+
+    @property
+    def type_name(self) -> TypeName:
+        return "mad"
+
+    def evaluate(self, a, b, c):
+        return a * b + c