Writing plug-in displays
Overview
Plug in displays are designed to sit in the bottom of scrolling spectrogram 
windows and display time series data which are not suitable for overlaying over
the spectrogram display itself. 
plug in displays take their time axis directly from the spectrogram display 
so that whatever is displayed is time aligned with the spectrogram data. 
To create plug in displays, the developer must create two separate classes
- DisplayPanelProviders
are used within PAMGUARD to make a list of possible display
panels that may be incorporated into other displays. 
 
- DisplayPanels
provide a graphics panel 
that is incorporated into the display, taking it's time information from
the display.