public SDDocumentType clone() throws CloneNotSupportedException {
        SDDocumentType type = (SDDocumentType) super.clone();
        type.docType = docType.clone();
        type.inheritedTypes.putAll(inheritedTypes);
        type.structType = structType;
        // TODO this isn't complete; should it be..?!
        return type;
    }
A document definition is a list of fields. Documents may inherit other documents,
implicitly acquiring their fields as it's own. If a document is not set to inherit
any document, it will always inherit the document "document.0".

@author Thomas Gundersen
@author bratseth
config-model/src/main/java/com/yahoo/schema/document/SDDocumentType.java