Settling-Velocity

Terminal settling velocity and relaxation time for aerosol single-particle dynamics. Migrated from Aerosol.jl@c8c640bb src/single_particle_dynamics.jl:113 (`SettlingVelocity` @component). Computes the terminal settling velocity v_t and the relaxation time τ (time constant for velocity adjustment) for a spherical particle falling under gravity in a viscous fluid (Seinfeld & Pandis 2006, Chapter 9, Eqs. 9.38 and 9.42).