Nextflow "set" input has been discontinued

As of Nextflow's version [19.08.1-edge](https://github.com/nextflow-io/nextflow/releases/tag/v19.08.1-edge), the set input type has been deprecated. Use tuple instead.

More info here: https://www.nextflow.io/docs/latest/process.html#input-of-type-set & https://www.nextflow.io/docs/latest/process.html#input-of-type-tuple