Auto-segment sequences if volume belongs to a Sequence node

[henrykrumb]

This PR addresses issue #53 by applying segmentations to all frames of a volume sequence. The following steps are executed:

1. If the selected volume belongs to a sequence, the user is prompted if they really want to segment the full sequence.
2. If the user declined, only the volume at the current frame will be segmented by TotalSegmentator (like it was done before this PR).
3. If the user accepted, all frames of the sequence will be iteratively segmented. A new SequenceNode will be stored in the SequenceBrowserNode of the input volume, recording the segmentation as a proxy node for every frame.

I tested my changes with the CT Cardio Sequence example on a vanilla Slicer 5.6.1 which confirmed that they work as expected. However, I would be happy for reviews.

[henrykrumb]

Quick demo of the auto-segmented CT Cardio Sequence example. I also tested it with the CTP Cardio Sequence example and it worked.

3dslicerheart.mp4

[lassoan]

Thank you, it looked good overall. I've touched it up a bit and will merge now.