fix secondary ID handling

[tomkralidis]

From OSCAR/Surface, for reference:

The OAI-PMH works with primary WIGOS IDs.

For the station 0-76-0-3170206000000036 the primary WSI is 0-20000-0-86776

So if you try:

https://oscardepl.wmo.int/oai/provider?verb=GetRecord&metadataPrefix=wmdr&identifier=0-20000-0-86776

you will get the file.

For the new system (OSCAR nextGen) we will make sure that WMDR XML files will be available also via non-primary WSI IDs.

Originally posted by @tomkralidis in #773

[tomkralidis]

Fixed in #14 (thanks @david-i-berry).