ENH: improve bundles QC by slicing 3 background planes at the center of mass of the bundle.

Hey gang! 

I think it would be helpful if `scil_viz_bundle_screenshot_mosaic.py` could offer an option to center the background 3 planes to be centered at the bundles center-of-mass (or any other metric that allow the planes to be better positioned).
For now, one gets eg. saggital slices with corpus callosum for intra-hemispheric bundles that do not intersect with that plane.

Thanks! 