Missing iocontext when cloning

Fix a bug in the Python wrapper for OpenZGY/C++ in the "clone" overload of the ZgyWriter constructor. The iocontext was not propagated. So this overload only worked on-prem.

Fixed and added a unit test.

A related note: There are defaults in place that pull the 3 mandatory settings from the environment. So in many cases an empty context is sufficient: iocontext=dict(). A completely missing context is still flagged as an error because it is likely that it reflects a bug in the program. That behaviour is debatable, but consider this: Often the context gets set up at a very high level, then gets passed down thru multiple layers. Forgetting to propagate iocontext at one point could cause a subtle bug because the application would sometimes work anyway. Just without any fine tuning (performance etc.) that the application thought it had requested.

This is exactly what happened here. Only the bug that was spotted was (eek!) inside OpenZGY itself.