Correct signal processing first pass

Wasn't properly handling non -1 values
Restructure first-pass to have rgbs be an atom too
Don't return 0 as a label
Check for if there are valid labels, not just prior-neighbours
Fix union find test expectation
Fix union bug of not padding enough sometimes
Remove debugging print from first-pass
Don't explicitly guard against finding the parent of 0
Implement second pass as signal processing operation
Replace first attempt at classical implementation
Relabel with minimum labels in the second pass
Reindent some stuff