Provide a graphics planel, or panels that can be added to the bottom of spectrogram
displays. The implementation of DisplayPanelProvider is only there to let Pamguard
know that panels can be created. On request, this will create any number
of vetoPluginPanels which will get added to the displays.
There are two basic ways that these things can update thier data. One is to
follow cue of the redrawing of the spectrogram, the other is to subscribe to
the some data block coming out of the detector. In this instance, we subscribe to
the backgroundDataBlock in the vetoProcess.
Note that we're handling several channels of data here !