Would be nice if this did real fluid dynamics

[smcameron]

In order to do so we need to fill in the advect_velocity_field() and remove_divergences() functions which currently do nothing. There are comments in the code describing roughly what they should do. And it's possible that what they describe isn't correct or won't work for whatever reason.

I'll need help for this one as it is beyond me. Might never get done.