Standardize headers and docstrings (#4417)

* Implement new headers

* Reformat 1

* Reformat 2

* Reformat 3 - math

* Reformat 4 - yaml

val.py

@@ -1,4 +1,6 @@
-"""Validate a trained YOLOv5 model accuracy on a custom dataset
+# YOLOv5 🚀 by Ultralytics, GPL-3.0 license
+"""
+Validate a trained YOLOv5 model accuracy on a custom dataset
 
 Usage:
     $ python path/to/val.py --data coco128.yaml --weights yolov5s.pt --img 640