docs: def _check_chn_type(channels, available_channels):
      """
      Function used for checking weather the elements in "channels" input are coincident with the
      available channels.
  
      ----------
      Parameters
      ----------
      channels : list [[mac_address_1_channel_1 <int>, mac_address_1_channel_2 <int>...],
                      [mac_address_2_channel_1 <int>...]...]
          From which channels will the data be loaded.
  
      available_channels : dict
          Dictionary with the list of all the available channels per device.
  
      Returns
      -------
      out : list
          It is returned a list of the selected channels in a standardized format.
  
      """
  
      # ------------------------ Definition of constants and variables -------------------------------
      chn_list_standardized = []
  
      # %%%%%%%%%%%%%%%%%%%%%%%%%%% Fill of "chn_list_standardized" %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      devices = list(available_channels.keys())
      for dev_nbr, device in enumerate(devices):
          if channels is not None:
              sub_unit = channels[dev_nbr]
              for channel in sub_unit:  # Each sublist must be composed by integers.
                  if channel in available_channels[devices[dev_nbr]]:
                      continue
                  else:
                      raise RuntimeError("At least one of the specified channels is not available in "
                                         "the acquisition file.")
              chn_list_standardized.append(sub_unit)
  
          else:  # By omission all the channels were selected.
              chn_list_standardized.append(available_channels[device])
  
      return chn_list_standardized