See https://github.com/jmhays/sample_restraint/releases/tag/v1.0.0-BRER and
Hays, J. M., Cafiso, D. S., & Kasson, P. M. Hybrid Refinement of Heterogeneous Conformational Ensembles using Spectroscopic Data. The Journal of Physical Chemistry Letters. DOI: 10.1021/acs.jpclett.9b01407