Merge packing folders

[valosekj]

This PR merges the packaging_lumbar_rootlets and packaging_ventral_rootlets folders into a single packaging folder to remove the number of duplicated scripts. By this, we removed duplicates of run_inference_single_subject.py and requirements.txt.

I still kept two READMEs to keep them easy to follow for collaborators. (inference instructions differ a little bit based on the type of the model)

• README_lumbar_rootlets.md
• README_ventral_rootlets.md

@KaterinaKrejci231054, once we have the MP2RAGE model ready to ship, you can add README_MP2RAGE_rootlets.md to the packaging folder.

Resolves: #66