Resolve "Hit amplitude not available when reading offline data"

CHANGELOG.rst

 Unreleased changes
 ------------------
+* The fields ``.a`` is now read by default.
 
 Version 0
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km3io/offline.py

@@ -41,6 +41,7 @@ class OfflineReader(EventReader):
             "dir_y": "hits.dir.y",
             "dir_z": "hits.dir.z",
             "tot": "hits.tot",
+            "a": "mc_hits.a",  # hit amplitude (in p.e.)
             "trig": "hits.trig",  # non-zero if the hit is a triggered hit
         },
         "mc_hits": {