make fractal a structure factor model

[pkienzle]

Currently fractal and core_shell_fractal are implemented as P(q) * S(q), where P(q) is the sphere structure factor for fractal or the core shell sphere for core_shell_fractal and S(q) is the fractal structure factor.

Propose splitting the S(q) into its own structure factor model so that all variants of fractal clustering are supported at once.

[butler]

This can be done in two pieces — treating the model as such in the background allows for cleaner reuse of code. PK says this has been done in his branch and should be implemented in 4.1. The second part is to present this to the user as independent structure factor. This may well be a good idea, but there is concern about how to distinguish different approaches to the fractal Sq and how users will know what to do. This requires some thinking and discussion before implementing so moving this to 4.2 for now

[butler]

Closed ticket #1168 which was essentially a duplicate. Some thoughts from that ticket:

Since all our fractal models are simply one of our form factors multiplied by the Teixeira Structure factor we should pull out the Teixeira fractal SQ and name it as such (i.e. teixera_fractal_sq). This distinguishes from other fractal models we may want to add. Eventually we may then want to remove the two/three existing combo fractals. 5.0 might the time to remove the old combos since that will break backward compatibility with saved projects and plugins that use the old models?