Update the sinking routines in WOMBATmid #71
Description
The idea is to use Rubey's equation for terminal velocity as a function of excess density and radius of a particle. The excess density is computed as a function of the organic and mineral content of the particle as well as the porosity. We use constant radii for small and large particles. We also must solve for the dynamic viscosity of seawater as part of the Rubey equation.