WIP: Resolve "Slicing of tracks"

Closes #15 (closed)

km3io/offline.py

 import uproot
+import numpy as np
 
 # 110 MB based on the size of the largest basket found so far in km3net
 BASKET_CACHE_SIZE = 110 * 1024**2
@@ -573,12 +574,14 @@ class OfflineTracks:
         if isinstance(item, int):
             return OfflineTrack(self._keys, [v[item] for v in self._values],
                                 fit_keys=self._fit_keys)
+        elif isinstance(item, list) and all(isinstance(i, str) for i in item):
+            cols = item
+            data = [getattr(self, c) for c in cols]
+            dtype = dict(names=cols, formats=[d.dtype for d in data])
+            return np.rec.fromarrays(data, dtype=dtype)
         else:
-            return OfflineTracks(
-                self._keys,
-                [v[item] for v in self._values],
-                fit_keys=self._fit_keys
-            )
+            return OfflineTracks(self._keys, [v[item] for v in self._values],
+                                 fit_keys=self._fit_keys)
 
     def __len__(self):
         try: