Fix issues in OpenSim::ExpressionBasedBushingForce::generateDecorations

[adamkewley]

Issue found when working on a student's model, which contained an ExpressionBasedBushingForce. The student had set the visual properties (visual_aspect_ratio, moment_visual_scale, and force_visual_scale) to zero in order to hide the frame/moment/force decorations that the component emits, but that resulted in NaNed geometry being emitted by the component, which then caused weird issues in opensim-creator's renderer.

When diagnosing that issue I also noticed that the function emits both of the frame decorations twice: once when fixed is true and once when fixed is false, and the NaN problem returns if the underlying implementation produces a NaN or zero-length vector (here, F_GM). The check against normSqr serves the double-purpose of checking for NaNs and zero-length results.

Brief summary of changes

Adds if checks etc. to the existing implementation to ensure that it filters out the edge cases.

Testing I've completed

Running it via OpenSim Creator

Looking for feedback on...

CHANGELOG.md (choose one)

• updated.

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This change is 

[nickbianco]