.NET framework 4.7.1 and newest C# version!

The version 4.7.x.x series of Altaxo is now based on .NET framework 4.7.1. This should not give any problems to users already using the 4.6.x.x series of Altaxo, since the .NET framework should be updated automatically (on Windows PCs).

Simultaneously, I also updated the Roslyn compiler to version 2.9. This makes it possible to use the newest C# 7.3 features in Altaxo scripts.