This class implements a step interpolator for the 3/8 fourth order Runge-Kutta integrator.

This interpolator allows to compute dense output inside the last step computed. The interpolation equation is consistent with the integration scheme :

• Using reference point at step start:
  y(t_n + θ h) = y (t_n) + θ (h/8) [ (8 - 15 θ + 8 θ²) y'₁ + 3 * (15 θ - 12 θ²) y'₂ + 3 θ y'₃ + (-3 θ + 4 θ²) y'₄ ]
• Using reference point at step end:
  y(t_n + θ h) = y (t_n + h) - (1 - θ) (h/8) [(1 - 7 θ + 8 θ²) y'₁ + 3 (1 + θ - 4 θ²) y'₂ + 3 (1 + θ) y'₃ + (1 + θ + 4 θ²) y'₄ ]

where θ belongs to [0 ; 1] and where y'₁ to y'₄ are the four evaluations of the derivatives already computed during the step.