ENH: added label save feature
ENH: added prompt for user name
ENH: structural and functional improvements

* separate helper functions into a separate class
* added support for storing settings (user name and results location)
* adding parameters to better maintain persistent info while switching between the steps
* initialization of the opacity and automatic linking of the nodes in the segmentation step
* minor changes to the segmentation step GUI
ENH: improved saving capabilities

* segmentation and w/l settings saved in the results dir
* added notification when save is completed