@Override
    public DocumentType getDataType() {
        return (DocumentType)super.getDataType();
    }
Sets the last modified time of this Document. This is typically set by the library that retrieves the
Document from persistent storage, and should not be set by arbitrary clients. NOTE: This is a
transient field, and will not be serialized with a Document (will be null after deserialization).

@param lastModified the last modified time of this Document (in milliseconds)
document/src/main/java/com/yahoo/document/Document.java