Add ability to only warp the translation part of an OpenSim::PhysicalOffsetFrame

[adamkewley]

Request from user testing the model warping workflow. The models they are throwing into the UI contain ("legacy") OpenSim::PhysicalOffsetFrames, which the warping algorithm can't generically handle (because the warping algorithm warps points, not rotation), so the user would like to be able to have shims like:

• Identity-warp offset frames (currently available)
• Only warp the position part of the offset frame (this ticket)
• Warp the offset frame following $STRATEGY (the future - i.e. users should ultimately be able to assign different strategies to different frames as the engine evolves)