Clone id handling #258

Currently the id of a Property, Section or a Document (and their children) remains the same if the object is cloned.

• Since these ids should be used only once in a Document, by default the id of the clone and all potentially cloned children should be set to new UUIDs.
• The id attribute has to be excluded in the baseobject.__eq__() method.
• The clone method should also contain a keep_id flag which by default is False. If set to True, the old id value is kept e.g. when attaching the exact same odML object to another Document.
• An additional validation check needs to prohibit the saving of a Document containing odML objects with identical ids.
• Currently ids of an odML entity cannot be set or changed after they have been created. Add a method new_id() to assign a new UUID to an odML entity analogous to the UUID assignment in the odML Entity __init__.