Assign the default cable length unit when not present (wireviz#206)

Bug: Failing to assign the default cable length unit when not present.
It was introduced in wireviz#198.

Fix: Test the correct cable attribute. This fix solves issue wireviz#205.

src/wireviz/DataClasses.py

@@ -228,7 +228,7 @@ def __post_init__(self) -> None:
             self.length_unit = u
         elif not any(isinstance(self.length, t) for t in [int, float]):
             raise Exception(f'Cable {self.name} length has a non-numeric value')
-        elif self.gauge_unit is None:
+        elif self.length_unit is None:
             self.length_unit = 'm'
 
         self.connections = []