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.

Co-Authored-By: Kirsten Sachs <kirsten.sachs@desy.de>
this must be an independent value, not the same as result. The outcome of the filter must be stored. finally the result yielded if the filter was positive after the loop. If you expect to yield several results from one loop there must first be a loop for the filter and a second loop for the results.