Add a delay in the Feature API fetch loop to avoid 429 errors

[ian-astun]

If you ask for a reasonable amount of data (say a county) python on my machine can exceed the (undocumented?) rate limit on even a relatively slow machine with a relatively slow broadband. So it would be useful to add a sleep step (preferably user configurable) to slow down the requests to prevent it just stopping.

[mattlovett]

Not a bad idea... but an even nicer solution would be to detect 429 responses and then sleep, so that you go as fast as you can until you hit the limit.