Add read/write support for YOLO txt files

[fracpete]

YOLO text file format:

• one text file per image
• one object definition per line:
  • <object-class> <x> <y> <width> <height>

Directory structure for output:

images/
  train/
  test/
  val/
labels/
  train/
  test/
  val/

Notes:

• object-class: 0-based index
• x/y: normalized center of annotation
• Normalization uses image width/height
• train/test/val in dir structure are the split names supplied as usual parameter
• also output (optionally):
  • a text file with labels (comma-separated list)
  • a CSV file with Index,Label columns

ADAMS implementations:

• read
• write