Add option to set reference arm in `g_km` #1682

All 774 tests pass, 21 skipped in 1m 25s

      1 files     81 suites 1m 25s ⏱️
  795 tests   774 ✔️   21 💤 0 ❌
1 688 runs 1 049 ✔️ 639 💤 0 ❌

Results for commit cd5ce1d.