Add SXRD tutorial

[aglavic]

Copied from SF ticket:

Hi,
I am trying to use Gen X to fit Specular (00L) crystal truncation rod data measured with varying incident (alpha) and detector angle. The available example file SXRD and SXRD2 shows instrument definition with fixed alpha angle. Could you please let use know how to change the instrument definition in SXRD for variable alpha?
I would be thankful for your help.

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SXRD is surface x-ray diffraction, that uses grazing incidence angles to measure with surface sensitivity. This is not possible on a (00L) reflection.

I will check if you could still use that plugin for your data. I assume the model will work for your case if you choose alpha large enought (e.g. >1 degree).