Raise error directing users to `add` function on `ModelRegistry` objects when `__setitem__` is invoked

[NeejWeej]

Is your feature request related to a problem? Please describe.
ModelRegistry objects do not have __setitem__ defined, but when this is attempted, an error should be raised that mentions the recommended way to modify the registry, namely using the add api here: https://github.com/Point72/ccflow/blob/main/ccflow/base.py#L409