Iteratively reweighted least squares

[casv2]

I think we have two usages for Iteratively reweighted least squares (IRLS) in mind, the first is to optimise any p-norm which @cortner will know a lot about. I think it'd be quite interesting to use IRLS to try and "even out" the relative error on the force components in the training database. After optimising with IRLS we'd have say 10% relative error on both large liquid and small vibrational forces hopefully resulting in both a good liquid rdf and phonon spectrum, without having to specify the weights.

[cortner]

+100