Add softseg support for nnUNet

[valosekj]

Originily posted in #13 (comment) by @tzebre:

About the naga comment for softseg support :

But, if we can find a way to encode the soft segmentations in a different range (not so 0-1) that can result in integer value after binarizing, yes, we can indeed use soft segs with nnuent. Hope this makes sense! (and it would really cool if we are able to do this!)

I have put the softseg segmentation support aside for the moment, it's going to take me a while. But I think we can merge without this feature for now.

[tzebre]

My first thought is to discretize softseg into integer value class (number of class given in args).
I need to check the maximum amount of different voxel value.

[tzebre]

I have created a new branch, the code is here

[valosekj]

I have created a new branch, the code is here

Awesome! 👍🏻 I took the liberty to open draft PR for easier code check/review.