Gets a description of the internal settings of the result producer,
sufficient for distinguishing a ResultProducer instance from another with
different settings (ignoring those settings set through this interface).
For example, a cross-validation ResultProducer may have a setting for the
number of folds. For a given state, the results produced should be
compatible. Typically if a ResultProducer is an OptionHandler, this string
will represent the command line arguments required to set the
ResultProducer to that state.