Hackathon External solution

In order to enforce external solutions onto a simulator, we need a way to specify the external solution and send it to the flow relaxation scheme (FRS) (open boundary condition) kernel.

Requirements is that the FRS kernel is invoked independently from the simulation step function (already in place), and but it currently assumes lake-at-rest external solutions only. By using the same approach with textures as we did with WindStresses, this could be achieved with no too much work.

The cool stuff we could make from this is e.g. simulations with tides.