Automatically use an image file as a segment group of the base DICOM.

[PaulHax]

If the user loads a DICOM CT and a foo.seg.nii file, there is no good way to automaitly load the .seg.nii file as a segmentation group. Right now the base CT name is arbirary:
https://github.com/Kitware/VolView/blob/main/src/actions/loadUserFiles.ts#L115
and the name from https://github.com/Kitware/VolView/blob/main/src/io/import/dataSource.ts#L145-L149