Determine gap between ray-tracing of chosen lamb on detector, due to non parallelism

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Motivation:

• In order to characterize the line doubling on spectra according to crystal's non parallelism, we must define a new method to compute the gap between few wavelength's arc on detector for few values of non paralllelism.
• This method has to take into account few values of alpha and beta angles for the entire crystal and splitted ones.

Proposition:

• Create a new method Crystal.gap_ray_tracing() to compute these gaps from Crystal_plot_line_on_det_tracing() method.
• Adapting this method to Crystal.split() to be the closest possible to the real detector.
• Return arc's gap as a plot with the detector's height as the y-axis.