by_id and by_index slices do not support scalar integers

[philsmt]

Unlike np.s_, our own special slices by_id and by_index do not support scalar integers to select a single train by ID or index:

• run.select_trains(np.s_[0])

  <extra_data.DataCollection for 81 sources and 1 trains>

• run.select_trains(by_index[0])

  ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  Cell In[130], line 1
  ----> 1 run.select_trains(by_index[0])
  
  File ~/code/EXtra-data/extra_data/reader.py:1091, in DataCollection.select_trains(self, train_range)
   1071 def select_trains(self, train_range):
   1072     """Select a subset of trains from this data.
   1073 
   1074     Slice trains by position within this data::
   (...)
   1089         If given train IDs do not overlap with the trains in this data.
   1090     """
  -> 1091     new_train_ids = select_train_ids(self.train_ids, train_range)
   1093     sources_data = {
   1094         src: srcdata._only_tids(new_train_ids)
   1095         for src, srcdata in self._sources_data.items()
   1096     }
   1098     files = set().union(*[sd.files for sd in sources_data.values()])
  
  File ~/code/EXtra-data/extra_data/read_machinery.py:135, in select_train_ids(train_ids, sel)
    133     return sorted(np.asarray(train_ids)[sel])
    134 else:
  --> 135     raise TypeError(type(sel))
  
  TypeError: <class 'int'>

• run.select_trains(by_id[run.train_ids[0]])

  ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  Cell In[131], line 1
  ----> 1 run.select_trains(by_id[run.train_ids[0]])
  
  File ~/code/EXtra-data/extra_data/reader.py:1091, in DataCollection.select_trains(self, train_range)
   1071 def select_trains(self, train_range):
   1072     """Select a subset of trains from this data.
   1073 
   1074     Slice trains by position within this data::
   (...)
   1089         If given train IDs do not overlap with the trains in this data.
   1090     """
  -> 1091     new_train_ids = select_train_ids(self.train_ids, train_range)
   1093     sources_data = {
   1094         src: srcdata._only_tids(new_train_ids)
   1095         for src, srcdata in self._sources_data.items()
   1096     }
   1098     files = set().union(*[sd.files for sd in sources_data.values()])
  
  File ~/code/EXtra-data/extra_data/read_machinery.py:135, in select_train_ids(train_ids, sel)
    133     return sorted(np.asarray(train_ids)[sel])
    134 else:
  --> 135     raise TypeError(type(sel))
  
  TypeError: <class 'extra_data.read_machinery.by_id'>