Nested element can be updated

[pmithrandir]

My code intend to correct few issues.

1
Nested elements might not be created / updated when the parent element is.
My code allow a mix of update and create element in the entire hierarchy.
A new element can contain an existing one for example.

2
In some cases, the self.fields_set variable doesn't contain some element. In my case, it was a list issued by a foreign key.
It might be a bug in pydantic, but in order to prevent that issue, I pushed a change to use fields instead.

3 The code is now also compatible with Optional elements without default value.
It was not working fully in the case a List element was optional, without a default value.(it was trying to insert None, leading to an exception

please tell me if my code needs improvements.

Regards,
Pierre

[Wouterkoorn]

This is quite a large chunk for me to review because I have to rethink why I made certain decisions, like choosing fields_set over fields.

Splitting the merge request up would help a lot. So each point you mentioned in it's own MR; same for spelling corrections.

Thank you for your understanding.