reflectometry_global.ipynb inequality_constraints.ipynb model_selection.ipynb lipid.ipynb batch fitting <https://nbviewer.jupyter.org/github/refnx/refnx-models/blob/master/batch/batch_fitting.ipynb> analytical SLD profiles <https://nbviewer.jupyter.org/github/refnx/refnx-models/blob/master/functional/functional.ipynb> nsf.ipynb