Add passive tracer advection-diffusion by a QG flow to traceradvectiondiffusion.jl

[jbisits]

I have been using a modified (by myself and Navid) version of the traceradvectiondiffusion.jl module to look at passive tracer advection-diffusion by a QG flow. Here is the repo for it:

https://github.com/jbisits/QG_tracer_advection

It uses the MultiLayerQG.jl module from GeophysicalFlows.jl to setup a multilayer QG flow and advect-diffuse the tracer using this flow.

If you think this would be worthwhile to add to this module (along with an example showing how it works) let me know and I can begin the process.

Joey

[navidcy]

It would be nice to enhance the module to allow this possibility. Let's think about what's the best way to do that.

cc @glwagner