`ERROR: LoadError: type DataType has no field mutable`

I'm having a similar issue to #18 while saving `PyObject` arrays. I have seen that in that issue they mention than deleting the environment solved it. I would very much like to avoid that, since messing with conda environments is always dangerous.

I'm just doing:
```
JLD.save(joinpath(ODINN.root_dir, "test/data/gdirs.jld"), "gdirs", gdirs)
```

Is this normal? What is the issue here?

Start with the JLD.save call shown in the issue and the `DataType has no field mutable` error, then compare the context and proposed resolution in issue #18. Reproduce the failure when saving the `PyObject` array; done means the cause is identified and saving works without deleting the environment.

Scritto dal modello di indicizzazione a partire dal testo della issue.