theta: $0<\theta\le 1$

Implicitness parameter for the free-surface solver and the vertial diffusion terms.

• $\theta=0$: Fully explicit (unstable)
• $\theta=0.5$: Crank-Nicolson (neutrally stable in the linear sense)
• $\theta=1.0$: Fully implicit
• $\theta=0.55$: For practical simulations