Return a *copy* (we hope) of the parameters.
        DANGER: Altering properties directly doesn't call model._cache

def params(self):
        """ Return a *copy* (we hope) of the parameters.
        DANGER: Altering properties directly doesn't call model._cache
        """
        params = odict([])
        for key,model in self.models.items():
            params.update(model.params)
        return params