Implement diffusivity limit for Harrington & Garaud prescription at high B

Limit finger velocity with drag limited velocity. This is calculated assuming that in the limit of very large B, Thermohaline Mixing approaches Rayleigh-Taylor. Then buoyancy and drag balance each other, which give a maximum velocity. Take v_th = Min(cs, v_max, w). Assume horizontal size $\ell_h$ for the calculation. Note that this assumes constant composition of the finger, and this assumption breaks down for fast fingers moving a long distance.