Missing test for xmile integrations may cause future bugs

[enekomartinmartinez]

It has been a while that several stateful objects builders used in SMILE2Py and vensim2py require the identifier as a first argument, for example add_delay. When the identifier was added in the builder, the elements from vensim2py build dictionary were updated. However, the elements from SMILE2Py weren't, as all the tests were passing.
I have added the identifier to all of them so they don't have bugs (it will be updated in PR #279), but, we should have a test for each builder in unit_test_xmile2py or integration_test_xmile.
I am not familiar with xmile and I don't have time to develop these tests, so please, if any xmile user can develop these tests it will be nice.

[JamesPHoughton]

@alexprey - do you have interest and time for updating the xmile parsing code? You're probably the one who knows it best at the moment. No worries if not. =)