OT t0 time tools provoke FP auditor into crashing

Gaudi::Math::ValueWithError's constructor and error methods are not transparent to quiet NaNs, since they do arithmetic on the constructor's arguments. Until we've figured out a good way to deal with that that impacts nobody else, we change the type returned by the OT track t0 tools (with a type alias) to something much simpler that is transparent to NaNs. Fix one place where a NaN produced by the OT track t0 tools confused the packing, while we're at it.

Phys/DecayTreeTupleTracking/src/TupleToolTrackTime.cpp

@@ -87,7 +87,7 @@ StatusCode TupleToolTrackTime::fill( const LHCb::Particle*,
     return StatusCode::SUCCESS;
   }
   
-  Gaudi::Math::ValueWithError track_t0 = m_trackT0Tool -> trackT0( *track ); 
+  const auto track_t0 = m_trackT0Tool -> trackT0( *track ); 
   
   // Fill tuple
   test &= tuple->column( prefix+"_TRACK_time",     track_t0.value() );