GET /flow/exists/{name}/{version}

Indicate whether a flow with a givern name and external version exists. If it does, return the flow id. If it does not, return a message that it does not.

Examples:

• Flow 7000 has name mlr.classif.rpart.tuned and external version R_3.4.0-v2.ddc1acf9. The endpoint returns {"flow_exists":{"exists":"true","id":"7000"}}.
• If instead there is no match, it returns {"flow_exists":{"exists":"false","id":"-1"}}.
• scikit-learn flows have names that are typically invalid in URLs, for example sklearn.ensemble.weight_boosting.AdaBoostClassifier(base_estimator=sklearn.tree.tree.DecisionTreeClassifier). These are supported by the POST endpoint by the same path where the parameter values are supplied through the request body: {"name": name, "external_version": external_version}. (See also #166).