Bind spectral axis ranges of residuals and data plots

[jbudynk]

Make sure that zooming in on one plot zooms in the same scale as the other plot along the spectral axis.

In Iris 2.1, the residuals spectral range updates when the data plot is zoomed in or out of. However, the data plot does not update when a user zooms in/out of the residuals plot.

Take care of possible issues in binding the residuals spectral axis to the data plot, in case there was a reason specview didn't update the data plot on residual plot changes (though no obvious reason pops up at the moment).