reindex subfield 6 occurrence numbers

Validator/fixer for reindexing $6 occurrence numbers. The order is based on the order of fields regardless of the tag. First field gets occurrence number 01, others get their value incrementally. Occurrence number '00' remains unchanged.

Reindexing will keep maximum occurence number small by removing gaps. This is needed/used by merge-reducers: source record occurrence numbers get big values to avoid overlap.