STCN: add printer database reader

[tijmenbaarda]

Pull request #53 adds the STCN person database, but the database of printers and publishers is equally important. According to Jeroen, they have to be separate readers.

[jgonggrijp]

Did Jeroen give a reason for the need to have them in separate readers?

[tijmenbaarda]

I don't remember well, but by making separate readers it will be possible to search them separately (without using complex query commands), and in addition, the data from these two databases are inherently different (even though both can be mapped to a BiographicalRecord)