Own ConvertManager and updating an existing object

Hi,

Here there is two patchs:
- The first one allow to use a custom convertmanager (one or multiple) and we can add different converter.
- The second one allow to instead create an object from the unflatted data, it updates an existing object.

There is missing a way to update an existing flatted structure in the same way i did with the unflatit
Not sure at 100% that my modifications are correct


Regards,