add binding option to chose between xsi:type and element based polymorphism in XML serialisation

The default until now has been to use xsi:type based polymorphism in XML serialisation - could add an option to do element based polymorphism - see [suggestion of how to do this in jaxb](https://stackoverflow.com/questions/20873523/jaxb-converts-xmlelementrefs-and-xmlelements-to-xschoice)

should really be a global choice?