Add hundred weight mass unit definitions

[rewdy]

This adds hundredweight units to js-quantities.

These are weird units, but are used in some specific industries.

• Definitions: https://en.wikipedia.org/wiki/Hundredweight
• Implementation in other unit conversion library (Python Pints): https://github.com/hgrecco/pint/blob/master/pint/default_en.txt#L579

[rewdy]

Any movement on this? Just checking in.