ENH: Rescale upon zooming in Stack1DMainWindow

[CJ-Wright]

Would it be possible to allow the aspect ratio of the Stack1DMainWindow viewer change so that when you zoom in on a peak/set of peaks it will spread out like a normal matplotlib plot? I am willing to PR I just don't know where to look/what to change.

[tacaswell]

I suspect it currently has fixed aspect ratio (so your pixel stay square). This is just using set_aspect method on the Axes to flip between 'equal' and 'auto' (irrc off the top of my head).

[CJ-Wright]

ok, thank you