Fix blank panels in reproject notebook preview image

xarray's plot.imshow sets aspect ratio based on coordinate extent.
When mixing degree-scale (EPSG:4326) and meter-scale (EPSG:3857)
panels in the same figure, the meter-scale panels get squeezed to
near-zero width. Fix by calling set_aspect('auto') on all subplot
axes so each panel fills its allocated space regardless of coordinate
scale.

Author: brendancol