Allow ZContext to be created using a user provided native context via IntPtr

[toeb]

For Inprocess Communcation between dotnet and native c/c++ the context pointer needs to be the same. If the C++ Backend provides this pointer then I can only inject it via Reflection into the ZContext. (by terminating then setting the private field in ZContext).

It would be nice to have a constructor or Factory Method which allows me to create the ZContext using an IntPtr as a parameter.

This could be dangerous of course....