CEI

I_P_DI_DDD2D - Simple continuity-equation-integrator.

I_P_DI_DDD2D integrates the continuity equation

dI __ __ __
-- = tau^-1*I - [vx vy]*\/I - \/*\/I + Q
dt
in 2D. I initial 2D concetration/intensity; DI increment; DT
time-step X and Y spatial coordinates, required to be very plaid;
TAU - lifetime, either scalar or the same size as I; D diffusion
coeficient, scalar; VX and VY wind, either scalars or the same
size as I, in which case compressible windfield can be modeled.