Update quantity repr

• [x] Closes #1495

• [x] Executed pre-commit run --all-files with no errors

• [x] The change is fully covered by automated unit tests

• [ ] Documented in docs/ as appropriate

• [x] Added an entry to the CHANGES file

• Update CHANGES

• Add test for repr

• Update repr function to include magnitude repr & units

any reason this wasn't merged for the last releases?

CodSpeed Performance Report

Merging #1579 will not alter performance

_{Comparing jules-ch:quantity-repr (b8e44aa) with master (62046f0)}

Summary

✅ 437 untouched benchmarks