New LengthUnitConversionRatio model class

New LengthUnitConversionRatio, an encapsulation of a pair of references (surrogates) to LengthUnits and a Ratio between the two, intended for use by LengthUnit to compare itself to other LengthUnits.

• Three properties: a Ratio instance, and two `id<LengthUnitish>` (taken and stored as surrogates, not owned).
	• Allows either or both LengthUnit ref to be nil (though reports invalid, of course).
• Normalish init methods: initWithRatio:fromLengthUnit:toLengthUnit:
• Manipulate methods to adjust the Ratio (manipulateRatioFromAToBAsDouble:) or swap out the LengthUnit(s) ().