Make the execmanager.retrieve_calculation idempotent as best as possible

[sphuber]

The retrieve_calculation will attempt to retrieve all the files once a calculation job has finished and store them in a FolderData node that is attached to the CalcJobNode with the retrieved link label. However, if the daemon runner is shutdown and is interrupted during this method, when it reload the process, it will call the same retrieve_calculation and will try to attach once again a node with the retrieved label. This will cause an exception, because the output label has to be unique. To solve it, the attachment of the FolderData should be done at the very end and a check should be added in the beginning for the existence of the output node, in which case it should be a no-op