extra SWIG warnings on DetectionPosition*

[KrisThielemans]

/home/sirfuser/devel/STIRTOF/src/include/stir/DetectionPositionPair.h:56: Warning 509: Overloaded method stir::DetectionPositionPair< unsigned int >::timing_pos() effectively ignored,
/home/sirfuser/devel/STIRTOF/src/include/stir/DetectionPositionPair.h:53: Warning 509: as it is shadowed by stir::DetectionPositionPair< unsigned int >::timing_pos() const.

This probably needs attributeref instead