Show salinity by color using a heatmap

This PR adds a salinity heatmap.

It is *astoundingly* ill-suited for this but worth throwing up here just because it wasn't much effort to get working and is interesting to look at.

![Zoomed in](http://i.imgur.com/G1CVcAc.jpg)

Or if you prefer pointillism:
![](http://i.imgur.com/nCCL7oK.png)