Support for SAXS fitting (v2)

[klytje]

Description

After long discussions on how to support SAXS fitting in SasView in the short term, it was decided to extend the generic scattering calculator with a 'SAXS fitting' option.

When this option is selected, the Compute button text is replaced with Generate plugin model. Loading a Nuclear data file with this option enabled circumvents the usual loading logic, and instead forwards the selected file path to AUSAXS. This serves two purposes:

1. AUSAXS also supports mmCIF files, which is the new standard in biological scattering, and
2. we avoid having to extend/modify the PDB loading logic with the additional information required for SAXS calculations.

Clicking the Generate plugin model will then create a new SAXS fit (<filename>) model, which is automatically selected & opened for the user in the fit panel. The SAXS data may then be loaded in & fitted.

Though a little awkward, I found this implementation to work decently well, at least for a short-term solution. In the long run, I'd like to have the following:

1. The ability to load both the SAXS data & structure file directly into the fitting panel (see my discussion here).
2. An optional initialization function for plugin models, which is invoked once before the actual Iq calculations start. I need this to avoid attaching the loaded structure file as a property to the Iq function. Having it float in the module namespace would make it load on SasView startup, which is also not ideal.
3. Custom plugin model interfaces in the fit panel. There's a lot of additional options available in AUSAXS which are currently difficult/impossible to expose to the user. Though this could also be implemented through a dedicated SAXS fitting window, for (1) to work, this is required.
4. We could potentially extend the SasView file loaders to support mmCIF & full PDB data loading, if we want to avoid relying on AUSAXS for this. Alternatively, we could continue to rely on AUSAXS to parse the files, but store the data on the SasView side. Or just keep it as is. I'm indifferent on this issue.

This PR replaces #3194.

Review Checklist:

[if using the editor, use [x] in place of [ ] to check a box]

Documentation
Do we need documentation for this? There's links to the AUSAXS paper in the generated plugin model file itself, if people are interested.

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)

[krzywon]

I haven't tested, but see my code comments.

[krzywon]

Do not merge: This PR needs to make a minor change to what is introduced by #3544 (due to an API-breaking pyAUSAXS version update).

Is this comment in your PR description still valid, @klytje?

[klytje]

@krzywon No, it is ready for merging. The shape2sas calculations were different from what I remembered and will not need manual updating (I just checked).

[krzywon]

One final code review. I'll do a functionality test later.

[krzywon]

Nitpick: It's often a bad idea to completely replace a method, in case there are direct calls to it that may have been missed.

Suggested change

	def change_computation_type(self):
	def change_is_avg(self):
	self.change_computation_type()
	def change_computation_type(self):

[klytje]

change_is_avg is a horrible function name, though. It does not at all indicate what it is actually doing. A quick check with grep indicates that the old name is not referenced in the code, so I think it is alright. Or do you prefer I change it back?

[krzywon]

How could a user arrive here and what should they do to correct it, if they do?

[klytje]

What about something like this? It's not critical that this part succeeds, which is why I wrapped it in a try/catch clause.
logger.warning("Could not bring the newly generated plugin model into focus in the Fitting window. Please report this issue.")

[krzywon]

I tested today and am getting a warning when trying to create the plugin model. I think this is a Win-specific issue. The path saved in the plugin model uses backslashes causing a unicode error.

When I open the file generated in the model editor, I get the following error:

2026-01-13 14:56:52,113 : WARNING : sas.sascalc.fit.models (models.py:166) :: Failed to load plugin '\sasview\SasView\plugin_models\ausaxs_saxs_plugin.py'. See \sasview\SasView\plugin_models\plugins.log for details
2026-01-13 14:57:07,276 : ERROR : sas.qtgui.Utilities.ModelEditors.TabbedEditor.TabbedModelEditor (TabbedModelEditor.py:448) :: Error building model: (unicode error) 'unicodeescape' codec can't decode bytes in position 2-3: truncated \UXXXXXXXX escape (, line 21)
2026-01-13 14:57:07,278 : ERROR : sas.qtgui.Utilities.ModelEditors.TabbedEditor.TabbedModelEditor (TabbedModelEditor.py:454) :: structure_path = "\sasview\src\sas\example_data\coordinate_data\diamond.pdb"