Multiple return properties are not completely supported

For example tFTPPut uses them and we have no way to do this.

We currently have a single "Return" property in ComponentProperties which is the parent of all return properties. However, we have no standard way to set these values in the RuntimeContainer. Currently we have the (rather lame) DATA_KEY_EXECUTION_RESULT as a single key for the return. We at least need a convention which allows the components to set each return value.

To make matters worse, the GDF-inspired Writer.close() which returns an arbitrary object (to be consumed by WriteOperation.finalize()) does almost the same thing.

We need to harmonize the GDF/DI approaches to this to satisfy both returns. Probably something along the lines of having the Reader.close() also return an object, and having a way for these return objects to give up their return values (by name) for the corresponding return properties.

This needs some more thought.