vesicle @ structure_factor incorrect

[pkienzle]

Comparing vesicle@S with no core to sphere@S I get different answers. Repeat using:

    $ sascomp vesicle@hardsphere,sphere@hardsphere radius=0,thickness thickness=40 -pars -midq -double

This is not true for hollow_cylinder with no core vs cylinder:

    $ sascomp hollow_cylinder@hardsphere,cylinder@hardsphere radius=0,thickness thickness=40 -pars -double

I think the issue is that the vesicle model defines volfraction, and this is colliding with the definition of volfraction in the structure factor, which then gets turned into volfraction_S.

Vescicle and sphere match if volfraction_S is set to the same value as volfraction.

    $ sascomp vesicle@hardsphere,sphere@hardsphere volfraction_S=0.05 scale=20,1 radius=0,thickness thickness=40 -pars -midq -double

Should update product model to use volfraction if it exists in the model.

This will require changes to sasmodels/product.py, both when constructing the model and when splitting the parameter set into P and S parameters.