Clone constructors a.k.a. introduce a __clone() method on some IObjects?

Some properties should not be cloned and need to be new instances on cloned instances (e.g. an identifier or UUID attributes). Basically everything that's set in `__construct()` should be set again in `__clone()`

This may not directly apply to `Document` or `DocumentType` as in the `field_path` branch, but is necessary for entities like `AggregateRoot` instances.
