Cleans up repo and adds script

[heyhaleema]

This PR:

• Cleans up/simplifies repo structure:

  XNAT/
  ├─ assets/ # logos, diagrams, etc.
  ├─ docs/ # SOPs, README, etc.
  ├─ tests/
  │  ├─ data/ # test data
  │  ├─ scripts/ # mirrors scripts folder
  ├─ xnat-csc/
  │  ├─ bash/ # bash scripts
  │  ├─  helpers/ # helper files, e.g. anonymisation .txt files
  │  ├─ scripts/ # .py scripts
  │  ├─ utils/ # functions abstracted from scripts/ if commonly used
  

• Adds script for ingestion reports

• Adds requirements.txt

• Updates .gitignore

• Moves send-to-pacs.py to xnat-csc/scripts (@laurencejackson FYI)