mmcif writer: refactor providing MMCifWriterEntity

For predictable behaviour, providing MMCifWriterEntity objects has
the following restrictions now:

- Only allow MMCifWriterEntity of type polymer
- All chains in ent that are of type polymer must be assigned
  to exactly one of these entity_info objects and must match the SEQRES
  (MMCifWriterEntity::mon_ids)
- All chain names that are assigned to entity_info must be valid chains
  in ent