Upgrade "Next Image" button

[TopHat327]

Currently, the system for resetting the program for a new image looks clunky and prone to errors. Research and update this to improve.

[TopHat327]

Idea: Create custom image object

Currently the program is written around singular image processing, so the image data is stored in globals and as components of the primary tkinter class (PDFViewer)(which is a terrible name btw). The image data should be removed to its own class to facilitate the progression of this program to work with multiple images. This would contain the base image, auto and manual masks, paint data, cell counts, etc.) This will also allow for better separation of the tkinter GUI data and funcs from the image data and funcs.