v5.17.0

What's new

• Added new units qr, U, and z in length.
• Added new units qr, fir, and us_qr in weight.
• Added new units dun, sec, and twp in area.
• Added new units st, fir, and pot in volume.

What's changed

• Changed implementation of #units_for! to raise error if the unit system is not defined
  within the unit group.
• Changed symbol of barleycorn length unit.