Add searchIndexProperties to allow serarching for DE numbers

[physikerwelt]

cf. #432

MaRDI Pull Request

Changes:

• Add searchIndexProperties to allow serarching for DE numbers

Instructions for PR review:

• [ ] Conceptual Review (Logic etc...)
• [ ] Code Review (Review your implementation)
• [ ] Checkout (Test changes locally)

Checklist for this PR:

• [x] Reviewers and Assignee specified.
• [x] All related issues are linked.