Resolve "Closing the files?"

Closes #47 (closed)

km3io/offline.py

@@ -323,8 +323,18 @@ class OfflineReader:
 
         """
         self._fobj = uproot.open(file_path)
+        self._filename = file_path
         self._tree = self._fobj[MAIN_TREE_NAME]
 
+    def close(self):
+        self._fobj.close()
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *args):
+        self.close()
+
     @cached_property
     def events(self):
         """The `E` branch, containing all offline events."""