Optional TesOutput

[peio-burucoa]

It would be great if TesOutputs could be marked as optional.

Indeed, there could be cases where tasks would be creating output files conditionally. In case of a missing output, the TES implementation would be responsible for setting the state to a failed state or COMPLETE, depending on the value of the optional property.

For instance, Nextflow provides the ability to tag some outputs as optional (see Nextflow optional outputs).