Integrate SIPrefixes with UnitComposition and Quantity

[JordanRL]

While there are some difficulties with this due to the fact that some units (anything that has mass) are based on kilograms instead of grams, the library should provide a way to scale to a specific SI prefix and adjust the units and unit display appropriately.

This will touch several parts of Quantity, and all of the naive*() methods in UnitComposition.