[Ifeffit] larch xray plugin - incorrect chantler data at edges

Hi, the larch xraydb plugin doesn't provide correct numbers for chantler f1 data around edges. Apparently this is due to incorrect interpolation in xray/xraydb.py in line 274. Below is a plot of "delta" from xraydb_plugin.xray_delta_beta() for Silicon which is terrribly wrong. The more correct "delta" as from chantler is shown in the following plot: --> I'd recommend to replace (line 274 in plugin xray/xraydb.py): out = UnivariateSpline(te, ty, s=smoothing)(energy) by a simple linear interpolation out = np.interp(energy, te, ty) The latter yields linear segments between chantler's data, but this is still better than the totally unphysical spline interpolation. Best regards, -- Johannes