add configuration to not cascade mapping

[mhgrove]

By default, we'll traverse the entire object graph serializing it into RDF, and serialize all of the RDF into the full object graph. There could be cases where you only want to serialize the top-level of the object graph and not cascade into its child objects. More often, during serialization to RDF rather than deserialization; during deserialization you can control this on your own by not providing any more than the top level of the object graph