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.
There is no function double.IsFinite() in .NET Framework.
Reworked the sanity check in Astronomy.Pivot so the C# code
builds in these older .NET platforms.
