Scale of mosacity color key can be wrong

The color key image scale is computed from the dimension range step (range[2]): https://gitlab.esrf.fr/XRD/darfix/-/blob/a8a46d051b56cf317927749a06614f0601d26346/src/darfix/core/grainplot.py?ref_type=heads#L191

But not only this value can be wrong (#167) but also the step value reflect the range of the motor positions and not of the center of mass which is used for the mosaicity plot.

The scale should then be computed from the center of mass range.