Use single source to check for radars

[peterdesmet]

There are currently two implementations to check for existing/valid radar codes:

• select_vpfiles() makes use of a helper function check_radar_codes()
• download_vpfiles() makes use of a helper function check_radar_codes()
• list_vpts_aloft() makes use of internal function

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• list_vpts_aloft() makes use of the radar file maintained at https://github.com/enram/aloftdata.eu/blob/main/_data/OPERA_RADARS_DB.json. That is a good file to use. I think the code should be moved to a helper function though.
• check_radar_codes() makes use of the lw-enram bucket, which will be deleted soon. That will break the functions depending on it. We should update this to make use of the new helper function.