Convert quantities to and from a UnitOf<TQuantity>

[tmilnthorp]

Adds UnitOf<TQuantity>, a unit of a quantity described by its names and conversions, without a unit enum value. Quantities can be converted to and from such a unit with no setup, and since the unit is tied to its quantity, using it with another quantity doesn't compile.

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

Length.FromMiles(1).As(furlong);     // 8
Length.Info.From(2, furlong);        // 402.336 m
Mass.FromKilograms(1).As(furlong);   // doesn't compile

• UnitOf<TQuantity> implements IUnitDefinition. It's created from the value of one unit in the base unit, or from conversion expressions for units with an offset, such as temperatures. Optionally, it 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.
• QuantityInfo<TQuantity, TUnit>.From(value, UnitOf<TQuantity>) returns the quantity in its base unit, since a quantity can still only be stored in one of its own units.

This is purely additive; no existing API changes. It's a first step toward units that UnitsNet doesn't define being usable like built-in ones. Parsing, formatting and storing a quantity in such a unit aren't covered yet. The subset that v5 supports is backported in #1739, so code written against it keeps compiling on v6.

Related: #1246, #1562, #1200

🤖 Generated with Claude Code

https://claude.ai/code/session_012XKhDsyHDc5BHrmibxScqG

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98%. Comparing base (3bf1ef4) to head (f3eaa8f).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1738   +/-   ##
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  Coverage      98%     98%           
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  Files         515     516    +1     
  Lines       24092   24115   +23     
======================================
+ Hits        23692   23715   +23     
  Misses        400     400           

Flag	Coverage Δ
net48	97% <100%> (+<1%)	⬆️

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