Allow to only deserialize selected types and ignore the others

[PythonFZ]

Currently if _type is there it will try to deserialize and fail. Sometimes it could be useful to only serialize a specific type.

see https://github.com/zincware/ZnTrack/blob/4551776c4397a1fc12b82829cbf3ee292faeea55/zntrack/fields/zn/__init__.py#L316-L320
where we only would like to serialize the converters.