Add support for single operator nodes in YAML dataflow specification

[phil-opp]

Single operator nodes are specified through an operator field, in contrast to the operators list of normal runtime nodes. Since there is only a single operator on such nodes, specifying an additional operator ID is not necessary. If no operator ID is given, the coordinator will assign the ID op.

Other nodes can also reference outputs of single-operator nodes by only specifying the node ID, the operator ID is optional here as well. The coordinator will automatically resolve these links.

This feature is useful for Python operators because all operators on the same node use a single global interpreter lock (GIL). By using a separate node for each Python operator, they run in different processes without GIL contention.