Add relatives attribute to PandasBlock.

cml/ports/source_adapters.py

@@ -42,6 +42,7 @@ class PandasBlock:
 
     def __init__(self, data_block, relatives=None):
         self.__data_block = data_block
+        self.relatives = relatives
 
     def __str__(self):
         return str(self.__data_block)