Based on the success of adding the mesh calls to the Python interface I wanted to add them to the C interface as well.

This version adds

• Access to face data in PDAL using the GetMeshSize and getAllTriangles methods,
• tests for the above
• Updates the sample C# P/Invoke scripts to add the Mesh functions and remove dependencies on Unity. Added .csproj files and improved some signatures
• added more examples to the readme.

I can merge this myself but I wanted to give time for code review first if possible.