Migrate recordIdentifier: rosdok/ppn123 to rosdok_ppn123

[rsteph-de]

We 've decided to replace the '/' in our recordIds with an '_'.
The slash proved complicated under some technical circumstances. (Part of URL, Part of Filename)

As a first step we replace all occurences of '/' with '_' whereever it is used in our code.
After the migration of code and data is completed we can clean up all these replace statements.

Attention!!! in PURLs wee keep the '/' as separator between projectID and localID