Improve ray-tracing with Rocking Curves data

[adriendaros]

• Using diffraction patterns obtained from the compute_rockingcurve() method in tofu/spectro/_rockingcurve.py, the aim is to plot the diffracted intensity for a given range of glancing angles.
• According to these, the maximum intensity, and even the whole pattern, should be shifted from the reference Bragg angle related to a given wavelength.
• A modification of the plot_line_on_det_tracing() routine in tofu/geom/_core_optics is preferred in order to make a good distinction between the Bragg angle of reference and the diffracted trace, and the same for several wavelength, compared to the 4-plots made by the cryst.plot() method.

[Didou09]

Remember @adriendaros : we close only after an official release includes the PR, in the meantime we leave it open and just add the label fixed in devel