fixing cable units bug from rotorD to km

[gbarter]

Purpose

Per error report via email, the cable length calculation was reported in km, but was computed in terms of number of rotor diameters and never scaled by rotor_diameter_m

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

• I have run existing tests which pass locally with my changes
• I have added new tests or examples that prove my fix is effective or that my feature works
• I have added necessary documentation