Corfunc parameter extraction is not correctly identifying the long period

[smk78]

Copied from PR #2450.

The Lamellar Parameter Extraction procedure in corfunc is not correctly identifying the Long Period. Transforming the example data ISIS_98929.TXT you get something like this:

Clearly, the 'true first maxima' lies around 75 Ang, but the extraction process is focussing on the wiggle about 30 Ang:

These data are from polyamide-6 so the actual long period is known (between ~60 - ~75 Ang). So at the moment the perspective is returning an inaccurate estimate. This is further reinforced by simple inspection of the position of the peak in the Q Space data: around 0.067 /Ang. Doing 2.pi/0.067 gives ~ 93 Ang. Almost 3 times the long period being returned at present.

The Lp should, I think, always be the first positive maximum (if the background correction has been correctly applied).

[lucas-wilkins]

Yikes, this could be a real issue, and a longstanding one, as I've not really changed this aspect of the calculation. Maybe we can have a chat about this.

[lucas-wilkins]

@smk78 I guess there is the following question: is the feature extraction wrong, or was the extrapolation done wrong, introducing wobbles.

It might be good to have something that somehow quantifies any error introduced by extrapolation.