Implement isotropic peak broadening to isotropic pseudo-Voigt profile

[clemisch]

It already exists in the anisotropic pseudo-Voigt profile in objcryst.

I added it to the isotropic profile because

• the effect is isotropic
• isotropic pseudo-Voigt is the only profile currently exposed to pyobjcryst

This broadening model is very limited and the resulting P value converted to apparent crystallite size should be taken very qualitatively.

It might still be useful to have this model to get a good fit, if you're sure observed peak broadening is coming from small crystallites, as the 2θ dependence is very different to existing per-phase Caglioti params U,V,W (which otherwise might be used to account for such effects empirically).

All of this to say: I understand if this addition is deemed not sensible, but for me is has been useful in practice, so here it is.

[clemisch]

Ah, there is a difference to the anisotropic profile: it uses P to broaden only the Gaussian component (because it has a split pseudo-Voigt with Gaussian U,V,W and Lorentzian X,Y params).

The isotropic PV profile only has shared U,V,W for FWHM and the mixing parameters Eta*. So here, the P term broadens the shared FWHM, which makes its way into Gaussian and Lorentzian components.

...this means it's sensible in practice to activate Eta refinement if P is active.

Maybe, ultimately, it's better to expose the more powerful anisotropic PV to pyobjcryst?

[vincefn]

Yes, I think it'd be better to expose the anisotropic PV to pyobjcryst - it can still be used without activating the anisotropic terms.