More work getting MacOS build process to work.
Avoid excessive number of floating point digits of
output in the demo tests, so that insignificant
floating point variations don't cause unit test failures.
Now that Microsoft has officially released .NET 6,
I have upgraded the C# version of Astronomy Engine to use it.
No source code changes were needed. I just bumped the
version number in the project files, and targeted .NET 6
in the GitHub Actions continuous integration tests.
Fixed some obsolete wording in generate/README.md.
