vendor example datasets

[dcherian]

It would be good to add the example datasets needed to xarray-data but in the interest of quick progress, I think it's ok to just hit some URL directly.

[dcherian]

I went ahead and did this: pydata/xarray-data#36 hehe

[keewis]

the raster example url has changed since then (see #34), so this has become more relevant again