add tests for the web API

This PR closes the circle and allows all bits of ruaumoko to actually be
tested. Aside from a simple little test of the web API which compares it to
using Dataset directly, this also teaches the web application about the
different tile sizes used in production and testing.

After this PR, I *think* that I'm almost done with ruaumoko and I can move on
to the predictor proper. :grin:

[Aside: the tile size thing is a little ugly and, in the fullness of time, some
sort of index/record of the tile sizes should probably be stored somewhere by
the downloader but that's not urgent. I'm loathe to change/add to the on-disk
representation since it would require a re-download on the actual ruaumoko
server.]