Add example showing how to sample gridded data at points

[shoyer]

It would be good to show how to do this, even if we don't have an efficient implementation yet (#214).

[nfaggian]

I read through #214 and noticed the use of a kd-tree might be a bit off for lat/lons. In the past I have had to transform coordinates first. Worth thinking about.

[shoyer]

That's a good point, thanks for bringing it up. Is pyproj still the best bet for that?

[nfaggian]

I think pyproj would be great to use.

[max-sixty]

Closing as stale, please reopen if still relevant