ReferenceError: weakly-referenced object no longer exists

[rsgalloway]

Wrapping a cask object from a PyAlembic IObject without creating a cask.Archive first can cause reference errors, e.g.

>>> iarch = alembic.Abc.IArchive(filepath)
>>> iobj = iarch.getTop().children[0]
>>> obj = cask.Xform(iobj)
>>> obj.is_animated()
...
    return self.parent.time_sampling_id
ReferenceError: weakly-referenced object no longer exists

This is because cask walks to the top to get the time smapling information, and the iarchive doesn't exist, or is weakly referenced.

A temporary fix is to pass in the time sampling id when instantiation the object, e.g.

>>> obj = cask.Xform(iobj, time_sampling_id=2)

A better fix might be to instantiate the archive on demand, or create a strong reference to the archive, if possible. I'm labeling this as an enhancement because this is an unexpected workflow in cask, but it'd be good to support.