See issue #1

`labeled(self)` may crash when the whole "Z" column is neither empty nor of type String (i.e. a number). Added prior casting.

conML/ports/source_adapter.py

@@ -297,7 +297,7 @@ class PandasBlock:
                 True, if labeled.
 
         """
-        values_empty = self.df["Z"].iloc[0].strip('""') == ''
+        values_empty = str(self.df["Z"].iloc[0])strip('""') == ''
         values_unique = self.df["Z"].nunique() == 1
         if values_empty and values_unique:
             return False