Postmodeling: Missing Parameter in Example Config

The [postmodeling tutorial](https://github.com/dssg/triage/blob/master/src/triage/component/postmodeling/contrast/postmodeling_tutorial.ipynb) uses `params.thresholds_iterator` in performing error analysis, however this parameter is not present in the [example postmodeling config file](https://github.com/dssg/triage/blob/master/example/config/postmodeling_config.yaml)