Produce delta objective function for boostrap
References
R Niebecker, MO Karlsson. (2013) Are datasets for NLME models large enough for a bootstrap to provide reliable parameter uncertainty distributions? PAGE 2013. https://www.page-meeting.org/Abstracts/are-datasets-for-nlme-models-large-enough-for-a-bootstrap-to-provide-reliable-parameter-uncertainty-distributions/
