It might be useful to allow the user to overlay more than just the target
source extraction regions in order to judge contamination from other sources in
the field of view.
A list of source IDs could be given and looped internally, plotting each adiitional region
with a different colour to the target source and an ID subscript. -NOW
IMPLEMENTED-
It might also be useful to add an aditional axis Channels describing the channels
used to construct the spectrum so that this plot can be compared to the
spectral plots produced by rgsspecplot.
