Problem 1451

Velocity of neutron is as same as incident velocity after hadron elastic process, such as H(n,np).
In G4HadronElasticProcess.cc (4.9.6) / G4WHadronElasticProcess.cc (4.9.5),
energy and momentum is calculated but velocity is not.
But in G4ParticleChange.cc,
// --------------------------------------------
if (!isVelocityChanged) theVelocityChange = pStep->GetTrack()->CalculateVelocity();
// --------------------------------------------
will apply velocity improper.
This problem may have same origin as problem 1275.

Above problem can be fixed by calling following code in G4HadronElasticProcess::PostStepDoIt.
// --------------------------------------------
 G4double mass = dynParticle->GetMass();
 G4double T = efinal/mass;
 G4double velocity = c_light*std::sqrt(T*(T+2.))/(T+1.0);
 theTotalResult->ProposeVelocity(velocity);
// --------------------------------------------