VD17 reader should merge Exemplar and Signatur fields into Holdings

[jgonggrijp]

Holdings information in VD17 is spread over two fields: "exemplar", which names the institution, and "signatur", which holds the signature. Currently, only the latter field is copied over into EDPOP's holdings field, but they should be merged.

[tijmenbaarda]

Unfortunately the data returned by the SRU server does not consistently provide the institution. We should probably contact the maintainers of VD17, and until then document it as part of #16.

[jgonggrijp]

Jeroen was able to provide an example where the institution is present in the original record but not copied by the explorer.

[tijmenbaarda]

Not closing this issue, but removing the 1.0.0 milestone because this is not likely to be fixed in the current project iteration.