## Draft: Advection Diffusion Model

I'm not sure if this is something we would really want, but I set it up for a different module and mentioned this in issue #1001 (closed).

This is the same as the tracer1p model with a stationary velocity field, but it does not assume that the domain is a porous medium. It should solve the transport equation decoupled from any momentum balance.

The test calculates a velocity field, passes this to a spatialParams, then calculates transport on the same domain with a fixed velocity field. To make it interesting, it's a rectangular domain with a circle cut out of the center.

This should produce the same result as the tracer1p model with the same velocity field and a porosity of 1.

If this is something we do want to include, It would likely benefit from a bit of refactoring.