core: fix missing namespace for BOOST < 1.60.0

9 jobs for pr/1348 in 146 minutes and 48 seconds (queued for 2 seconds)
Name Stage Failure
failed
format Test
-    MotionPlain __plus__(const MotionRef<Vector6ArgType> & v) const
- { return MotionPlain(m_data+v.toVector()); }
-
- template<int O2>
- MotionPlain __minus__(const MotionTpl<Scalar,O2> & v) const
- { return MotionPlain(m_data-v.toVector()); }
-
- template<typename Vector6ArgType>
-
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