Modify the policy to handle reference feature.

include/sot/core/feature-1d.hh

include/sot/core/feature-abstract.hh

include/sot/core/feature-generic.hh

include/sot/core/feature-joint-limits.hh

include/sot/core/feature-line-distance.hh

include/sot/core/feature-point6d-relative.hh

include/sot/core/feature-point6d.hh

include/sot/core/feature-task.hh

include/sot/core/feature-vector3.hh

include/sot/core/feature-visual-point.hh

src/feature/feature-1d.cpp

src/feature/feature-abstract.cpp

src/feature/feature-generic.cpp

src/feature/feature-joint-limits.cpp

src/feature/feature-line-distance.cpp

src/feature/feature-point6d-relative.cpp

src/feature/feature-point6d.cpp

src/feature/feature-task.cpp

src/feature/feature-vector3.cpp

src/feature/feature-visual-point.cpp