kernel_slowness_to_velocity Converts from slowness to velocity parameterizations
 
  G : kernel [1,nkernels]
  V : Velocity field (
 
 
  CALL:
    G_vel=kernel_slowness_to_velocity(G,V);
  or 
    [G_vel,v_obs]=kernel_slowness_to_velocity(G,V,t);
  or
    [G_vel,v_obs,Cd_v]=kernel_slowness_to_velocity(G,V,t,Cd);