SYSML21 — Declaration of SpatialItem::coordinateFrame is incorrect

SpatialItem in standard library SpatialItems.sysml is defined by ThreeDCoordinateFrame[1]. However, this is too wide as it may not be any ThreeDCoordinateFrame but should be narrowed to ISQSpaceAndTime::Spatial3dCoordinateFrame[1] since it must be a spatial coordinate frame that is a measurement reference for 3D position or displacement vectors.

Furthermore, in subclause 9.7.2.2.5, SpatialItem::coordinateFrame is defined by the even wider VectorMeasurementReference and misses the default value. This should be corrected in line with the above, i.e., ISQSpaceAndTime::Spatial3dCoordinateFrame[1] , and adding the default value.