Feature dotnet wrapper

[mortensp]

1. DDS_Core added as a wrapper for .Net applications.
2. DDS_Core_Demo contains examples using most of the methods in dll.lib via the new wrapper.
3. In dds I have added dds_api.h with 'EXTERN_C DLLEXPORT' on the the SolverComtext methods.
4. In the docs is a new dotnet_interface.md file deskribing the new wrapper and a link is made in the ReadMe.md file.
5. The build warning on Directory.Build.props has been totally eliminated.
6. I'm considering making a Nuget package for the wrapper, but for that to work on MacOs i'll need a MacOs version of dds_native from someone:-)

Note: I have not been able to build using Bazel, so everything has been done i VS.