Allow saving one file for all label vs different files for each label

[maxradx]

Is your feature request related to a problem? Please describe.
Currently, SlicerCART saves all label in one file and do not offer the option to the user to save segmentatino masks in one or multiple files.

Describe the solution you'd like
Option selection for saving one file for all label vs different files for each label

[AcastaPaloma]

Do we only want to give the following two options:

1. All labels
2. Every single label separately

Or do we give the user to choose which labels?

[maxradx]

The user should have the option to chosse!
Almost done.
Assigned myself to the issue :)