C#: More benefits from using netstandard2.0.

Now that I have retargeted astronomy.csproj from
net5.0 to netstandard2.0, there are a couple of
other little improvements that are now possible:

1. In my manual Framework 4 test project, instead
   of directly pulling in the source file astronomy.cs,
   add astronomy.csproj as a project reference.
   This demonstrates that the same binary astronomy.dll
   works in both Framework and Core.

2. Now there is no need/use for conditional compilation
   directives in the Astronomy.CubeRoot function.
   Instead, always use my own implementation since the
   Math.Cbrt function is never available.
   From a testing standpoint, this was probably
   the better option all along.

generate/dotnet/framework4/AstronomyTest/AstronomyTest.csproj

@@ -43,14 +43,17 @@
     <Reference Include="System.Xml" />
   </ItemGroup>
   <ItemGroup>
-    <Compile Include="..\..\..\..\source\csharp\astronomy.cs">
-      <Link>astronomy.cs</Link>
-    </Compile>
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
   </ItemGroup>
   <ItemGroup>
     <None Include="App.config" />
   </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\..\source\csharp\astronomy.csproj">
+      <Project>{7c3fb2a1-2996-4ad0-9d47-52a4227ea78f}</Project>
+      <Name>astronomy</Name>
+    </ProjectReference>
+  </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
 </Project>

generate/dotnet/framework4/AstronomyTest/AstronomyTest.sln

@@ -5,6 +5,8 @@ VisualStudioVersion = 16.0.32106.194
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "AstronomyTest", "AstronomyTest.csproj", "{0611FB45-451F-4770-9864-1F34A4609B3D}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "astronomy", "..\..\..\..\source\csharp\astronomy.csproj", "{7C3FB2A1-2996-4AD0-9D47-52A4227EA78F}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -15,6 +17,10 @@ Global
 		{0611FB45-451F-4770-9864-1F34A4609B3D}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{0611FB45-451F-4770-9864-1F34A4609B3D}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{0611FB45-451F-4770-9864-1F34A4609B3D}.Release|Any CPU.Build.0 = Release|Any CPU
+		{7C3FB2A1-2996-4AD0-9D47-52A4227EA78F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{7C3FB2A1-2996-4AD0-9D47-52A4227EA78F}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{7C3FB2A1-2996-4AD0-9D47-52A4227EA78F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{7C3FB2A1-2996-4AD0-9D47-52A4227EA78F}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

generate/dotnet/framework4/README.md

@@ -1,6 +1,9 @@
 # .NET Framework 4+
 
-Astronomy Engine officially supports dotnet core (.NET 5+).
+The C# version of Astronomy Engine is targeted to
+[.NET Standard 2.0](https://docs.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-2-0).
+Therefore, it supports .NET Framework 4+ and .NET Core 5+.
+
 Currently I do all unit testing and development under .NET 6.
 However, I make an effort to maintain backward compatibility
 with .NET Framework 4+, although issues may slip through.

generate/template/astronomy.cs

@@ -8030,12 +8030,11 @@ $ASTRO_IAU_DATA()
 
         private static double CubeRoot(double x)
         {
+            // Astronomy Engine is targeted at .NET Standard 2.0.
+            // That means it supports the older Framework 4+ platform
+            // as well as .NET Core 5+.
             // .NET Core has a Math.Cbrt function, but .NET Framework doesn't.
-#if NET
-            // Use the standard Math.Cbrt where available.
-            return Math.Cbrt(x);
-#else
-            // Provide a substitute cube root function when Math.Cbrt isn't available.
+            // Therefore, I have to implement my own cube root function.
 
             if (x < 0.0)
                 return -CubeRoot(-x);
@@ -8044,7 +8043,6 @@ $ASTRO_IAU_DATA()
                 return 0.0;
 
             return Math.Pow(x, (1.0 / 3.0));
-#endif
         }
 
         /// <summary>

source/csharp/astronomy.cs

@@ -9242,12 +9242,11 @@ namespace CosineKitty
 
         private static double CubeRoot(double x)
         {
+            // Astronomy Engine is targeted at .NET Standard 2.0.
+            // That means it supports the older Framework 4+ platform
+            // as well as .NET Core 5+.
             // .NET Core has a Math.Cbrt function, but .NET Framework doesn't.
-#if NET
-            // Use the standard Math.Cbrt where available.
-            return Math.Cbrt(x);
-#else
-            // Provide a substitute cube root function when Math.Cbrt isn't available.
+            // Therefore, I have to implement my own cube root function.
 
             if (x < 0.0)
                 return -CubeRoot(-x);
@@ -9256,7 +9255,6 @@ namespace CosineKitty
                 return 0.0;
 
             return Math.Pow(x, (1.0 / 3.0));
-#endif
         }
 
         /// <summary>