Switch to float32

Apply me on top of #46

Motivation: less disk space => better caching.

I very much doubt we need the precision of doubles in our wind data;
there are far greater sources of noise elsewhere.

Incidentally, it looks like the wind data we get is rounded to 2 or 3dp
anyway by the NOAA. (Difference between data and [data rounded to 2dp]
has mean 5e-5, max 0.005.)