add rof weights generation script

[anton-seaice]

As described here add a script to make remapping weights which map runoff from land cells to ocean cells and conserve the total volume of runoff. This will use the esmf mesh format used by the OM3 mediator.

[aekiss]

FYI here's the runoff misdirection problem we found previously with a kdtree method when the nearest ocean point is on the other side of a long skinny land barrier COSIMA/access-om2#130

[anton-seaice]

Thanks!

It's possible a brute-force approach rather than a KD-tree would be fine - we only have to make the weights once. The brute-force approach would allow for a more accurante surface of a sphere distances in metres rather than euclidean (in degrees). It also might automate some cleverer handling of the nearest cell to preference cells connected through the ocean, rather than over land (although it would take some thinking to know how to include the JRA & deseas landmasks etc).