Add particle advection example

[jbisits]

This PR adds an example where we are advecting particles in a flow. I was thinking about #2 and thought that if we just set diffusivity to be zero we are solving the advection equation so could maybe use that in lieu (or until) of having some kind of module that advects parcels. It is a little rough and ready but interested to see if you think it worthwhile.

[navidcy]

When I run the example I get this...

particle-advection.mp4

It's probably tricky to work without diffusivity due to numerical instabilities?

[jbisits]

When I run the example I get this...

particle-advection.mp4
It's probably tricky to work without diffusivity due to numerical instabilities?

Yes I did not think the one I ran locally was quite that bad, but the lack of diffusivity does cause problems with numerical instabilities. More seeing if you thought worth pursuing but I think best if I remove this example altogether. If I just close this PR does it get archived but not merged?

[navidcy]

Yeap, that's what will happen if you close.

[navidcy]

It'll stay in your fork unless you delete that branch.

[jbisits]

Ok I think I might do that just in case some inspiration strikes.. Thanks for you help and sorry for the bother with this!