Update comments to include description of the base IMU name and its heading axis options.
Update comments to include description of the base IMU name and its heading axis options.

opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core