assert that the arity of this setter matches the fields given.
assert that the arity of this setter matches the fields given. if arity == -1, we can't check, and if Fields is not a definite size, (such as Fields.ALL), we also cannot check, so this should only be considered a weak check.
Mixed in to both TupleConverter and TupleSetter to improve arity safety of cascading jobs before we run anything on Hadoop.