BibCheck: allow filtering by subfield contents
- Add optional argument subfield_filter to iterfields() method that accepts a (code, value) pair. If a code is passed then only fields matching this pair will be yielded. (PR #2474)
Co-authored-by: Kirsten Sachs <kirsten.sachs@desy.de>
Tested-by: Tibor Simko <tibor.simko@cern.ch>