Support reading an attribute from a class instead of calling a method?

This is a discussion issue as a followup #71

Should we natively support returning a class attribute value (or multiple) instead of calling a method of the class during the produce phase?

This could be simple achieved at the JSON level by simply not specifying the method key.

Then, we would have:

    "produce": {
        "output": [
            {
                "name": "feature_weights",
                "attribute": "feature_importances_"   # This would be optional, to be used only if the used name does not match the attribute name.
                "type": "list"
            }
        ]
    }

Start by reading follow-up issue #71 and tracing the produce-phase configuration described here. Determine how method-based outputs are currently represented and whether omitting method can identify a class attribute. Done means the proposed attribute form and its optional name mapping have a clear, agreed implementation path.