Enrich DBS records with Sigel data based on the DBS ID

- exclude NULL values in DBS records
- exclude Sigel records without DBS reference
- restore DBS ID (`inr`) for JSON output
Transform Sigel field names and values, add links

Based on transformation by @dr0i (https://github.com/lobid/lodmill)
Wrap literals that can appear multiple times into entities

Use `name="<name>[]"` syntax to output as arrays in JSON
Transform DBS fields and combine as entities with Sigel fields