Problem with NetworkUnit backend

[heplesser]

While exploring NetworkUnit based on this example, upon instantiating C = C_sim(name='C') and error occured here

https://github.com/INM-6/NetworkUnit/blob/d882c55ea9321523553a3fafcb79d2df19536906/networkunit/models/backends/storage.py#L19

since self.model.unpicklable is undefined according to Python.