nlohmann::json and VTK's public API

[dcthomp (David Thompson (Kitware))]

@ben.boeckel

My head hurts when I try to think about how this would get misused (i.e., it would only ever work as a local variable because as a member “who owns the actual thing” sounds like a nightmare to reason about). But I also don’t think that only supporting bigobj[innerbigobj] as a full copy of the inner object makes sense either. This will need documentation, probably along the lines of “use auto when storing lookups, vtkJson in objects, and use vtkJsonRef at API boundaries” or something.

1. My thought was to mitigate this in cases where the JSON data being referenced was owned by a vtkObject subclass; we could have vtkJSONRef own a reference to the VTK object that owns the JSON data.
2. I agree we would still need to warn developers in cases where JSON data was not owned by a vtkObject:

   You are responsible to ensure the underlying JSON object’s lifetime extends beyond that of every vtkJSONRef that references it.