ff17b44a41
An SGRawBase class has been added as a base class to the SGRawValue hierarchy so that SGPropertyValue functions don't necessarily need to know the type of the value stored in the node. A new SGRawValueContainer class stores properties that shouldn't be stored in the node itself. PropertyTraits indicates if a type is stored in the property node or externally. Add getValue and SetValue template member functions to SGPropertyNode. Read and write new extended properties. Rearrange props.hxx a bit so that the template magic actually works. Split out extended raw value virtual functions into a seperate base class. SGRawExtended is chosen as a base class of SGRawValue for extended property types.
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