MAINT: Add validator to validate_column.

Who validates the validators?

zipline/pipeline/pipeline.py

@@ -229,6 +229,7 @@ class Pipeline(object):
             raise AssertionError("Unknown graph format %r." % format)
 
     @staticmethod
+    @expect_types(term=Term, column_name=str)
     def validate_column(column_name, term):
         if term.ndim == 1:
             raise UnsupportedPipelineOutput(column_name=column_name, term=term)