Add docstring stub

Author: Klaus Zimmermann

esmvalcore/preprocessor/_io.py

@@ -247,6 +247,7 @@ class _TrackedCube(NamedTuple):
 
         @classmethod
         def from_cube(cls, cube):
+            """Construct tracked cube."""
             times = cube.coord("time")
             start, end = times.core_points()[[0, -1]]
             return cls(cube, times, start, end)