Skip downloading when not in pre-sampled coords

[tatsubori]

SSL4EO's download_ssl4eo.py can fail with NoKey error if given pre-sampled coords like sampled_locations.csv do not contain any index specified by the indices range.

[adamjstewart]

@wangyi111 may be interested in reviewing this

[wangyi111]

yes I think if we skip then a warning is good. Normally, the indices range for download_ssl4eo.py is supposed to be covered by sampled_locations.csv. So the warning can help the user check again if this is really needed.

One rare use case could be you want to filter out some coords in sampled_locations.csv (e.g. you want to download more images within an area), move those entries to another sampled_locations_special.csv file for download. In this case, you still specify the full indices range in sampled_locations.csv to download_ssl4eo.py, and the script can skip those ids that are not in the new csv.

[calebrob6]

I'd recommend returning False if skipped and returning True if completed, then at the end of the script printing how many were skipped.

[adamjstewart]

I added a warning message. Adding return values and summing them is more work, but @calebrob6 can add another commit/PR if he wants.