Rename property attribute @ref to ref?

[jesper-friis]

When representing DLite properties with Python dataclasses or pydantic models, the @ref attribute create some trouble since it is not a valid Python identifier. Although pydantic v2 support aliases, it is an unnecessary complication and it is not supported in dataclasses, pydantic v1 and other similar framework.

The simplest solution is simply renaming the attribute from @ref to ref.

Are the any good reasons not to do so?

For backward compatibility, we could keep support for @ref in the json parser and instance_from_dict() for a while together with a deprecation warning.