WIP: Resolve "uproot4 integration"

Closes #58 (closed)

km3io/offline.py

 from collections import namedtuple
+import logging
 import warnings
 import uproot
 import numpy as np
@@ -7,6 +8,8 @@ import awkward as ak
 from .definitions import mc_header
 from .tools import cached_property, to_num, unfold_indices
 
+log = logging.getLogger("offline")
+
 
 class OfflineReader:
     """reader for offline ROOT files"""