add a default relation to capture "output resources" of a workflow step

[lsteinmann]

A WorkflowStep done on a resource may produce a new resource. This needs to be recorded with a relation. Ideally the output resource would be visible in the list of WorkflowSteps.

Let's say we have a Sample, which in our case is a soil sample. We do flotation on it (which is a WorkflowStep "Flotation"). Now this produces archaeobotanical remains, or pottery sherds, or other Find or FindCollection resources. These need to be linked via a relation to the WorkflowStep.

Similarly, restoration WorkflowSteps may yield new objects, e.g. a complete Stone block or a vessel.