[FEATURE] Change "Label" to "SegmentationMask" for clarity

[Linardos]

Is your feature request related to a problem? Please describe.

It gets a bit confusing that GaNDLF uses the word "label" to refer to segmentation. Reason I'm proposing this change is because I spent some time into the code (specifically in the forward_pass.py) where I expected the label_present to refer to classification until I realized it label refers to segmentation in the current GaNDLF version.

Describe the solution you'd like

Instead of Label it would help clarity if instead GaNDLF used the term SegmentationMask both in the csvs and throughout the codebase
I understand the change might run deep but it would make it more intuitive for new users as label is widely associated with classification