Allow different nuclear and magnetic models for a single fit to a single data set

[butlerpd]

Is your feature request related to a problem? Please describe. Currently SasView models (strictly speaking sasmodels) assume as single underlying structure (sphere, core-shell sphere, rectangle ect.) which has a nuclear contrast component and a magnetic contrast component. This is often not the case however. The simplest case might be a single nuclear sphere with a core-shell magnetic structure, but one might imagine exotic systems.

Describe the solution you'd like What would be ideal would be to use the amplitudes of the existing models to construct a composite model of say a rectangular magnetic core embedded in a large rectangular (or spherical?) particle with a single nuclear SLD

Describe alternatives you've considered This could be done by building a series of bespoke models such as the one by @dehoni currently on the marketplace https://marketplace.sasview.org/models/136/ (Superparamagnetic Core-Shell Spheres with 3D field orientation). This however only makes sense if the universe of such bespoke models would be small.

Additional context This will require a new sasmodels construct which should be designed generally so it can be used eventually to build new shapes based on existing shapes. This would be the most general approach. It goes without saying .. but let's say it: polydispersity needs to be included separately for the magnetic and nuclear models

From the sasview GUI side

• This requires that mag scattering be accessible to 1D fitting (currently only 2D can turn on magnetic terms)
• This also requires models that can handle 1D cuts (discussed separately?)
• The simplest approach would likely be to add a second drop down model box when magnetic scattering is checked. One drop down would be labelled nuclear and the other magnetic
• With the number of parameters it will be important to have a simplified constraint interface. For example one would want to constrain nuclear_radius = core_mag_radius+ mag_shell_thickness. The current approach available for single tab constraints might be sufficient but needs to be verified

NOTE: This is meant to supersede #2487, #2490, and #2489

[butlerpd]

Copied from #2487 by @dehoni

As an expansion, one could allow to select different basic form factors (spheres, cubes, etc) from a drop-down list also for the magnetism tab in the SASview GUI. This would already create some flexibility. To describe the magnetic response with field, you need some information/or sophisticated guess about the system. The above-mentioned model is IMHO the simplest problem with analytical known solutions. One might change to superball (continuous transition from cube to sphere).