Support time-dependent land masks

[spencerahill]

I have just come across this use case for the first time: the land mask is defined at each timestep, i.e. (time, lat, lon). We currently assume that the land mask is defined only in (lat, lon). The time-dependent case is more general and so we should support it.

I don't foresee this being massively difficult; it just throws off some of our existing logic a bit. Will put together a PR hopefully relatively soon.