Better file names for custom urls

[cholmes]

Follow on from #31 - the urls for custom downloads are just 'custom_download_<date/time>'. We should be able to use the file name being downloaded if it's a single parquet file, which is often the case with custom downloads. Not sure if there's any good way to get the name for a glob () url of many parquet files, as often it'll have a 'field_name=', and often in a nested structure.