Generic math for UnitsNet in .NET 7 (#1164)

Co-authored-by: Andreas Gullberg Larsen <andreas.larsen84@gmail.com>

Changes:
- build UnitsNet in .NET 7 as well with very limited conditional compilation for the generic math interfaces
- extended IQuantity interface hierarchy to support generic math (as the TSelf parameter for CRT pattern, and the underlying value type are needed)
- automatically generated operator overloads and parsing methods are covered by the respective generic math interfaces

Still to do:
- also cover the manually added operator overloads in the *.extra.cs files with generic math interfaces
- also implement ISpanParsable, which will need some work, especially in the QuantityParser
- maybe for symmetry reasons also overload the unary + operator and implement IUnaryPlusOperators. However, I'm unsure because it's a pretty useless operator
- resolve nullable reference types chaos: since multi-targeting with net7.0 even when disabling the nullability in the csproj I kept getting tons of errors for both netstandard2.0 and net7.0 builds, so for now I've disabled the respective warnings. We should fully use the nullable reference types feature in the whole code base also for .netstandard2.0 to solve this tangle

Pain points:
- multiplication with one operand of Quantity type and one operand of double/decimal type only supported with Quantity being the first parameter (when adding the commutative case, I get errors 
![grafik](https://user-images.githubusercontent.com/30859615/206543723-613058a3-d3ea-4232-ae42-e37fe812ab5f.png)
- I suspect we'll run into the same issues with some custom overloads involving commutative multiplication with different quantity types

Author: AndreasLeeb

CodeGen/Generators/UnitsNetGen/QuantityGenerator.cs

@@ -64,7 +64,7 @@ namespace UnitsNet
             Writer.WLIfText(1, GetObsoleteAttributeOrNull(_quantity));
             Writer.W(@$"
     [DataContract]
-    public readonly partial struct {_quantity.Name} : IQuantity<{_unitEnumName}>, ");
+    public readonly partial struct {_quantity.Name} : {(_quantity.GenerateArithmetic ? "IArithmeticQuantity" : "IQuantity")}<{_quantity.Name}, {_unitEnumName}, {_quantity.ValueType}>, ");
             if (_quantity.ValueType == "decimal")
             {
                 Writer.W("IDecimalQuantity, ");
@@ -239,9 +239,19 @@ private void GenerateStaticProperties()
         ///     Gets an instance of this quantity with a value of 0 in the base unit {_quantity.BaseUnit}.
         /// </summary>
         public static {_quantity.Name} Zero {{ get; }}
+");
 
-        #endregion
+            if (_quantity.GenerateArithmetic)
+            {
+                Writer.WL($@"
+        /// <inheritdoc cref=""Zero""/>
+        public static {_quantity.Name} AdditiveIdentity => Zero;
 ");
+            }
+
+            Writer.WL($@"
+        #endregion
+ ");
         }
 
         private void GenerateProperties()

UnitsNet.Tests/GenericMathExtensionsTests.cs

@@ -0,0 +1,37 @@
+// Licensed under MIT No Attribution, see LICENSE file at the root.
+// Copyright 2013 Andreas Gullberg Larsen (andreas.larsen84@gmail.com). Maintained at https://github.com/angularsen/UnitsNet.
+
+#if NET7_0_OR_GREATER
+using UnitsNet.GenericMath;
+using Xunit;
+
+namespace UnitsNet.Tests;
+
+public class GenericMathExtensionsTests
+{
+    [Fact]
+    public void CanCalcSum()
+    {
+        Length[] values = { Length.FromCentimeters(100), Length.FromCentimeters(200) };
+
+        Assert.Equal(Length.FromCentimeters(300), values.Sum());
+    }
+
+    [Fact]
+    public void CanCalcAverage_ForQuantitiesWithDoubleValueType()
+    {
+        Length[] values = { Length.FromCentimeters(100), Length.FromCentimeters(200) };
+
+        Assert.Equal(Length.FromCentimeters(150), values.Average());
+    }
+
+    [Fact]
+    public void CanCalcAverage_ForQuantitiesWithDecimalValueType()
+    {
+        Information[] values = { Information.FromBytes(100), Information.FromBytes(200) };
+
+        Assert.Equal(Information.FromBytes(150), values.Average());
+    }
+}
+
+#endif

UnitsNet.Tests/UnitsNet.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net6.0;net48</TargetFrameworks>
+    <TargetFrameworks>net7.0;net48</TargetFrameworks>
     <LangVersion>latest</LangVersion>
     <IsTestProject>true</IsTestProject>
     <NoWarn>CS0618</NoWarn>