Refactor img_viewer

[dagewa]

Changes intended to improve readability and maintainability of this long module.

• Break long functions apart into smaller units
• Reduce nested scope levels
• Remove many try blocks that were obscuring the code paths

There's still much to do in here, but I'd like to request review now and continue later if this is merged. Testing should mainly concentrate on the mask generation as this is where most changes were made.

[dagewa]

This is intended to address #122

[dagewa]

For future reference it isn't necessary to merge into master for testing. You could have just run git checkout refactor-img_viewer to switch to the branch, and git checkout master when you are done.