.NET bindings

We (Cegal) have a branch with basic C and .NET bindings to the (new) OpenZGY API.

• netstandard2.0, with xunit tests and a console app for net5.0
• Capable of building Debug and Release nuget packages
• Console app can dump ZGY meta data, copy and compare ZGY content
• Windows only for now (I would need assistance for Linux support)
• Seismic Store support - sdapi binaries and headers are added to branch
• 2 source files are added to OpenZGY.vcxproj to provide the C bindings, no changes to existing code
• Bindings expose a simplified API for now (e.g. no compression or const support)

Would this be of interest for this repo? Until more complete, it should probably live as a separate branch.