Implement diffusivity limit for Harrington & Garaud prescription at high B

[matteocantiello]

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.

[evbauer]

(edited above to fix up a couple of notation things to avoid later confusion)

Just noting here also that current velocity w and horizontal size lhat that we have calculated are non-dimensionalized, so we'll need to be careful to do unit comparisons before calculating the maximum velocity.

[afraser3]

Note that length scales are non-dimensionalized by d = (kappa_T * nu / thermal_brunt^2)^(1/4), and velocity scales are non-dimensionalized by kappa_T/d. And that lhat is a wavenumber so it has units of 1/d.