Support partial unit systems (#1690)

Extracted from #1544 because representing a unit system for only the
dimensions an application uses is useful independently of QuantityValue.
Moving it out gives the API change focused review and reduces the
original PR's scope.

Changes:
- Allow UnitSystem to be constructed from any BaseUnits value defining
at least one dimension.
- Continue rejecting null and fully undefined base-unit sets.
- Clarify the constructor documentation and exception message.

Tests:
- Change the seven previously rejected one-dimension-missing cases to
assert successful construction.
- Add a single-dimension UnitSystem case.
- Add a dedicated assertion that BaseUnits.Undefined remains invalid.

Author: angularsen

UnitsNet.Tests/UnitSystemTests.cs

@@ -34,11 +34,20 @@ public void ConstructorThrowsArgumentNullExceptionForNullBaseUnits()
         [InlineData(LengthUnit.Meter, MassUnit.Kilogram, DurationUnit.Second, ElectricCurrentUnit.Ampere, null, AmountOfSubstanceUnit.Mole, LuminousIntensityUnit.Candela)]
         [InlineData(LengthUnit.Meter, MassUnit.Kilogram, DurationUnit.Second, ElectricCurrentUnit.Ampere, TemperatureUnit.Kelvin, null, LuminousIntensityUnit.Candela)]
         [InlineData(LengthUnit.Meter, MassUnit.Kilogram, DurationUnit.Second, ElectricCurrentUnit.Ampere, TemperatureUnit.Kelvin, AmountOfSubstanceUnit.Mole, null)]
-        public void ConstructorThrowsArgumentExceptionWithUndefinedUnits(LengthUnit? length, MassUnit? mass, DurationUnit? time, ElectricCurrentUnit? current,
+        [InlineData(LengthUnit.Meter, null, null, null, null, null, null)]
+        public void ConstructorSupportsPartialDimensions(LengthUnit? length, MassUnit? mass, DurationUnit? time, ElectricCurrentUnit? current,
             TemperatureUnit? temperature, AmountOfSubstanceUnit? amount, LuminousIntensityUnit? luminousIntensity)
         {
             var baseUnits = new BaseUnits(length, mass, time, current, temperature, amount, luminousIntensity);
-            Assert.Throws<ArgumentException>(() => new UnitSystem(baseUnits));
+            var unitSystem = new UnitSystem(baseUnits);
+
+            Assert.Equal(baseUnits, unitSystem.BaseUnits);
+        }
+
+        [Fact]
+        public void ConstructorThrowsArgumentExceptionWithUndefinedUnits()
+        {
+            Assert.Throws<ArgumentException>(() => new UnitSystem(BaseUnits.Undefined));
         }
 
         [Fact]

UnitsNet/UnitSystem.cs

@@ -16,11 +16,11 @@ public sealed class UnitSystem : IEquatable<UnitSystem>
         /// <summary>
         /// Creates an instance of a unit system with the specified base units.
         /// </summary>
-        /// <param name="baseUnits">The base units for the unit system.</param>
+        /// <param name="baseUnits">One or more base units that define the unit system.</param>
         public UnitSystem(BaseUnits baseUnits)
         {
             if (baseUnits is null) throw new ArgumentNullException(nameof(baseUnits));
-            if (!baseUnits.IsFullyDefined) throw new ArgumentException("A unit system must have all base units defined.", nameof(baseUnits));
+            if (baseUnits == BaseUnits.Undefined) throw new ArgumentException("A unit system must define at least one base unit.", nameof(baseUnits));
 
             BaseUnits = baseUnits;
         }