docs: Add a docstring for the ScanHeader class

src/pye57/scan_header.py

@@ -5,6 +5,10 @@
 from pye57.utils import get_fields, get_node
 
 class ScanHeader:
+    """Provides summary statistics for an individual lidar scan in an E57 file.
+
+    Including the number of points, bounds and pose of the scan.
+    """
     def __init__(self, scan_node):
         self.node = scan_node
         points = self.node["points"]