Improved helicorder plotting (all-in-one plot, etc.)

- robust to changed channel names due to metadata change
- outputs all channels in ONE plot
  (first plots individual channels, then merges them)
     !! REQUIRES pillow package !!
- new optional parameters:
    * `equalize` (bool): equalize waveforms (counteract impulsiveness)
    * `shifts` (list): list of times to shift waveforms; useful to maintain
      alignment if detections were reassociated to other templates
    * `chnlidxs` (list/tuple): indicate which channel to plot BY INDEX
      (e.g., `[1, 2]` only for horizontal ones)
- COULD plot in parallel, but hardcoded to serial processing (`n_processes = 1`)
- minor fixes