Add KPM to main

[Lazersmoke]

This feature is under active development by Harry, so I assume it's not ready yet. This PR is so that it is not forgotten!

[Lazersmoke]

Also: per a discussion with Martin today, a third kind of intensity formula, KPMIntensityFormula will need to be added, since KPM responds to S(q,w) queries instead of (disp(q),intensity(q)) queries. I can create this from the KPMintensities and KPMdssf functions if/when they are stabilized.

[kbarros]

Also: per a discussion with Martin today, a third kind of intensity formula, KPMIntensityFormula will need to be added, since KPM responds to S(q,w) queries instead of (disp(q),intensity(q)) queries.

I am not sure about the details, but this sounds familiar to the situation from classical dynamics -- in both cases, we will be querying S(q,w) for a list of q values. Could the UI be shared?

[Lazersmoke]

in both cases, we will be querying S(q,w) for a list of q values. Could the UI be shared?

The UX is shared (in the sense that its just intensity_formula in all cases), but they are doing fundamentally different things.

Classical: list of indexes describing qs from the list of available qs. If a q is outside that list, then interpolation needs to happen, intensities_interpolation

KPM: list of any, arbitrary qs, with no interpolation. Then, we wouldn't want users to call intensities_interpolated because that's misleading. So it should probably be called either plain intensities or intensities_kpm if we don't want people confused why intensities(::StructureFactor) isn't defined. (c.f. LSWT uses intensities_broadened and intensities_bands depending if there is broadening)

What can maybe be done is to define intensities_binned(::KPM) (but only if we can analytically integrate the KPM Intensity(q,w) function over small finite volumes (q+dq,w+dw), @hlane33 ?), and actually this would be highly desirable for inverse problem if its fast.

[Lazersmoke]

FYI on this: Probably the Cairo dependency at least should be removed before merging. That's just for the examples, which can go in SunnyTutorials instead presumably? Or ported to GLMakie and left here

[kbarros]

Before we merge this I think @hlane33 should reapply the commits so that they are correctly credited as the author, with coauthor ddahlbom.

(To deal with rebasing issues, I reapplied the changes from scratch, which accidentally gave myself credit.)

[kbarros]

Development for this feature has moved to #289.