What should happen if a cuds object is deleted on the remote side of the transport layer

[pablo-de-andres]

In GitLab by @urbanmatthias on Dec 10, 2019, 09:07

Consider the following scenario:

1. The user uses a transport layer to connect to a simulation session.
2. The user creates a CUDS data structure and calls run().
3. The cuds objects on the client side become "expired" (We do not want to send all the created data back to the user). Accessing an expired CUDS object will trigger a (lazy) reload of the cuds object.
4. The user accesses the an attribute of a cuds object, that has been deleted on the remote side.

Problem: The cuds object exists on the client side, but not on the server side.

What should happen?

Possible solutions:

1. An error should be raised. (Problem: How should the user know which cuds objects have been deleted?)
2. None should be returned. (Current solution. Problem #187)

What do you think @yoavnash ?