ReqIF-SDoc: exporting/importing (roundtrip)

[stanislaw]

• [x] Basic sections with requirements
• [x] SDoc custom grammars
• [x] SECTIONs with FREE_TEXT
• [ ] COMPOSITE_REQUIREMENT
• [x] How to deal with single vs multiline strings?
  • [x] Use a if "\n" in decision for now
• [x] Export parent-child relationships
• [ ] Export requirement-to-source-file relationships
• [ ] Parsing images