WTNR thumbnails: sigma clipping-based plotting

[Theodlz]

In this PR (feature request from @virajkaram) for WNTR thumbnails, we:

• set nans to the max value instead of the median, so masked portions of the image are more obvious on the difference image (these just vanish when setting nans to the median, as it is often a very low value for the difference image).
• use sigma clipping to get the vmin and vmax, which makes things a lot more obvious on all image types.