Merge `packaging_ventral_rootlets` and `packaging_lumbar_rootlets` folders

[valosekj]

Currently, the repo contains the following two folders:

• packaging_ventral_rootlets
• packaging_lumbar_rootlets

These folders contain instructions on how to use beta models (not part of SCT yet)

Their content is pretty similar; for example, run_inference_single_subject.py is almost identical for both folders. I believe I created two folders because I shared different models (cervical, lumbar) with different users. But it would be better to merge the folders into something like packaging to reduce duplicities.

TODO:

• wait for merge of Pediatric Rootlets #53