Visualize salinity contours

This PR is the final piece for the first draft of #16 and #9.

It builds on #36 and should be merged *after* that one.

The hardest part of this by far is getting the call to `contour` correct. Most things that I have tried have resulted in worthless rectangles in tiny bays along the coast and nothing else.

The good news is that the *plotting* side appears to work fine.