[v5] Convert quantities to and from a UnitOf<TQuantity>

[tmilnthorp]

Backport of the forward-compatible part of #1738 to v5. It adds UnitOf<TQuantity>, a unit of a quantity described by its names and its value in the base unit, without a unit enum value. Quantities can be converted to and from such a unit with no setup.

var furlong = new UnitOf<Length>("Furlong", "Furlongs", 201.168); // 1 furlong = 201.168 m

double furlongs = Length.FromMiles(1).As(furlong); // 8
Length length = Length.Info.From(2, furlong);      // 402.336 m, in the base unit

• UnitOf<TQuantity> optionally takes the base units it's made of, e.g. feet per day: new BaseUnits(length: LengthUnit.Foot, time: DurationUnit.Day).
• quantity.As(UnitOf<TQuantity>) returns the value in that unit. Using a unit of another quantity doesn't compile.
• quantityInfo.From(value, UnitOf<TQuantity>) returns the quantity in its base unit. It throws ArgumentException for the info of another quantity.

Purely additive, and the same call sites compile on v6, so code written against this keeps working after upgrading. v6 builds further on it:

• exact fraction values;
• units with an offset, such as temperatures;
• IUnitDefinition;
• compile-time checking for From;
• the groundwork for units UnitsNet doesn't define to be used like built-in ones, which is v6-only.

Related: #1738, #1246, #1562

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