[bug] Failing to assign the default cable length unit when not present

[kvid]

The current dev has a bug that fails to assign the default cable length unit when not present. The cause is this line:
https://github.com/formatc1702/WireViz/blob/606ddbf9775da6300299640501083acfaef42077/src/wireviz/DataClasses.py#L231
The wrong attribute is tested. The correct code line should be:

        elif self.length_unit is None:

I'm sorry that I approved #198 without detecting this bug. It was introduced in 694f3a3, and I suggested a modified version still containing the bug, that was applied in d9b67e4, but it was not detectable at run-time before removing the redundant code in e9292fb.

This is a good example of a bug that would have been detected automatically by a proper regression testing protocol (see also #63), e.g. something like:

python build_examples.py
python build_examples.py -b dev-x2 compare

Note that dev-x2 in the command above is a local branch where I have committed the generated files after merging in #70 with eebf932 (the latest commit where the output was intentionally changed).

[formatc1702]

Thanks.

I hope we can somehow implement a more rigorous and automated regression testing / unit testing soon; unfortunately I can't guarantee I will have the time and energy to dive deeper into this topic. I'll add a comment to #63 and mark it as "help wanted".