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

[graste]

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.