Add structure_factor results to Saxs module

As discussed on the discord channel there is some confusion what the Saxs (and the DiporderStructureFactor) module is actually calculating. This MR tries to make this more clear by renaming of the results properties and explicitly making the structure factor available. The new attributes and internal variables are renamed as

1. q -> scattering_vectors
2. structure_factors (new)
3. scat_factor -> scattering_intensities
4. q_indices -> miller_indices

I hope this is clear now?! I also updated the docs, the explanation around Saxs calculations and the Saxs example. For the example, I extended the plot to show the structure factor, the form factor and how the scattering intensity is constructed from these.

Let me know if this is understandable without a deep dive in the theory. 😆

PR Checklist

• Docs?
• [] Issue raised/referenced?