text: Title: Complex signal type warning
  Body: Hi everyone, I get the following warning while loading a hologram: 
  
  `sig1 = hs.load(s1, signal_type='hologram')`
  
  `WARNING:hyperspy.io: signal_type='complex_signal2d' not understood. See hs.print_known_signal_types() for a list of installed signal types or https://github.com/hyperspy/hyperspy-extensions-list for the list of all hyperspy extensions providing signals.
  `
  
  It is pretty annoying, since I load multiple files and the warning extends over multiple pages. In this list (hs.print_known_signal_types()), the signal_type appears.
  
  I need to load the signal as indicated above, otherwise I cannot call a few functions like:
  
  ```python
      sb_position = sig1.estimate_sideband_position(ap_cb_radius=None, sb='lower')
      sb_size = sig1.estimate_sideband_size(sb_position) * 2/3
      statistics = sig1.statistics(sb_position=sb_position)
  
      out_size = int(2*sb_size.data)
      wave_image = sig1.reconstruct_phase(ref1, sb_position=sb_position, sb_size=sb_size, output_shape=(out_size, out_size))
      wave = wave_image.unwrapped_phase()
  ```
  
  If I don't call them with the signal_type option, these functions are not available.
  What do I do wrong?