What did 3.1.2 do with polydispersity in the Fractal Model; what should 4.2 do?

[smk78]

Greg Newbloom & Adrian Rennie have both commented on the removal of polydispersity on the radius parameter in the fractal model in 4.0.1. It was allowed in earlier versions.

The fractal S(Q) in this model also includes the radius parameter (as well as the spherical P(Q)).

So there are two questions:

1. Did 3.1.2 and earlier versions only apply polydispersity to P(Q), or to P(Q) and S(Q)?

2. Should polydispersity be put back into this model for 4.2?

[butler]

Clearly my fault. The git record clearly shows that I converted this model and failed to make radius a volume parameter, while Piotr converted the fractal core-shell and DID make radius and thickness polydisperse. Clearly this was done as an oversight not deliberately due to some deep problem. I will correct this for 4.1

[butler]

In 478882278efdd578c67d41a151c1209805308f3f/sasmodels:

added polydispersity back to fractal (as it used to be, as the
documentation implies and as core_shell_fractal already has). Also
formatted documentation according to current standard for references and
authorship/verification. Fixes #877