Fix UNet interactivity UI

[michaels10]

The Unet threshold interactivity UI needs an overhaul. In particular:

• A preview should be automatically displayed upon specifying a model.
• Preview should be dynamic (use the threshold) by default.
• Preview should allow the user to change peak/FOV.

[michaels10]

OK, so I've taken a look at the work required. This fix will require the following steps:

• Pull the UNet prediction and the postprocessing out into separate functions.
• Prevent the user from changing the threshold before they specify the model.
• Upon specifying the model, auto-load the first peak (with the threshold as interactive)
• Allow the user to change FOV/peak within the preview.

[michaels10]

@rthiermann Could you pull out the postprocessing/unet analysis? It's a little wonky.

[rthiermann]

I've pushed an update with the pre and post-processing functions separated from the main segmentation.