I can not use eio clip when target area is too big #45

I found that eio clip causes error when target AOI is located in too many srtm grid. Do you happen to know how we can solve this?

`root@yuend:~/eio_test# eio clip -o srtm.tif --bounds 12.79 40.53 15.42 42.47

Traceback (most recent call last):
File "/usr/local/bin/eio", line 8, in
sys.exit(eio())
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 722, in call
return self.main(*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/local/lib/python3.6/dist-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/click/decorators.py", line 17, in new_func
return f(get_current_context(), *args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/elevation/cli.py", line 59, in wrapper
return wrapped(**kwargs)
File "/usr/local/lib/python3.6/dist-packages/elevation/cli.py", line 93, in clip
elevation.clip(bounds, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/elevation/datasource.py", line 181, in clip
datasource_root = seed(bounds=bounds, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/elevation/datasource.py", line 150, in seed
"for how to bulk download tiles." % len(ensure_tiles_names))
RuntimeError: Too many tiles: 12. Please consult the providers' websites for how to bulk download tiles.`