Allow users to save workflow artifacts

[gpelouze]

This is possible with argo, but we need to modify the workflow template to enable it.

We create a default workflow folder (eg. /tmp/workflow_output), and save anything that users add to this folder as workflow artifact. In the same way as we train them to use /tmp/data, we should tell them to add files to /tmp/workflow_output if they want to retain data after the workflow execution is done.