v1.7.0

Changelog

dd8d896 fix: engine: backoff was looping infinitely when reaching MaxElapsedTime without sleep (#145)
4ba4845 fix: engine: step with invalid JSON output are considered as FATAL_ERROR (#146)
2731c6e fix: error messages for bad requests
b3dc0c9 functions: new way to describe utask executors using other executor, with yaml (#135)
c4c2ac0 plugin: retrieve non-pointer interface before casting into plugin Interface