v0.1.3-alpha.1 - Cubic spline velocity function inversion

Cubic spline velocity function inversion (Development version of creVelocityInversion package)

• Changed

• Resolve #23

• Use previous spline function as input: Use the previous velocity cubic spline function as input
  for the next iteration in the velocity inversion algorithm. 675d7a7

• Use cubic spline velocity model in inversion instead of a constant velocity gradient model 15c7b55