decoder: rename iteration variables to improve readability

decoder: rename iteration variables to improve readability

Use longer variable names for the iteration over channels, options,
annotation classes and rows, and binary classes when a decoder gets
loaded. This improves readability, especially in nested loops with
complex test conditions and diagnostics messages.

Although the Python variables were checked for their availability and
expected data type, and we should only see non-negative index values,
the iteration variable's data type remains unchanged in this commit
(sticks with signed size types).