Add holding information to SRU readers

[tijmenbaarda]

Holding information is important for the users of EDPOP, but it is not available as part of the Marc21 records that the SRU API returns. Jeroen asked the people at the library in Göttingen, who manage the HPB SRU interface. They mentioned the following about HPB, which will probably equally work for the other SRU APIs:

The SRU-API of the HPB does allow retrieval of holdings information, but it is only contained in picaxml records (recordSchema=picaxml). There, you can find the information in field 009B.