keep zoom the same when changing between r and Halpha images in the main frame

[rfinn]

• when the program first starts, the Halpha image is displayed in the mainframe.
• the user can click 'r' to display the r-band image, and then 'h' to get back to Halpha.
• however, every time a new image is displayed, the zoom goes back to the full image. Ideally, the zoom will be the same as before changing the image, so that if I'm inspecting a galaxy in Halpha and want to see what it looks like in r, I don't have to recenter and rezoom.
• I tried to implement this

        if key == 'r':
            z = self.coadd.fitsimage.settings.get_setting('zoomlevel')
            print('zoom = ',z)
            p = self.coadd.fitsimage.settings.get_setting('pan')
            print('pan = ',p)

            self.coadd.fitsimage.set_data(self.r)
            self.coadd.fitsimage.zoom_to(z.value)
            self.coadd.fitsimage.panset_xy(p.value[0],p.value[1])
            self.coadd.canvas.redraw()
        elif key == 'h':
            self.coadd.fitsimage.set_data(self.halpha_cs)

Unfortunately, this doesn't work.

• this is an enhancement - it doesn't really hinder forward progress on research, so I'll leave this for a later date once the code is operational.