previews: respect aspect ratio step size

Some observations use a step size perpendicular larger than the resolution along the slit. This leads to skewed preview images, as the pixels would essentially need to be non square.

matplotlib supports this, but it also needs to be checked through the entire chain of calculations and plotting:

The preview plots should use the wcs keywords to determine the values for the aspect ratio and spatial extent of FOVs. Example: https://archive.sdc.leibniz-kis.de/SDCDetailServlet?Instrument=gris&ObjectId=607c9dcd18262e0426a2b2a2

Also see the corresponding issue on Jira