ModelObject#clone does not copy properties

[nicoweidner]

Currently, ModelObject#clone creates a clone of the instance it is called on inside the passed model store. It does not copy any properties of the original object. The properties of the new object will be:

• Empty in case the object did not exist already in the new store
• The corresponding properties from the new store if the object already existed in there

In the discussion in #106, it turned out that this may be unintended and properties should actually be copied over.
In that case, we should probably fail in case the object already exists in the new store, to avoid accidentally overwriting data (or pass a overwrite flag).

Note: The only usage of clone in the library seems to be inside a test, so changing it won't cause problems in the library. It could be considered a breaking change to public API, however...

[goneall]

@nicoweidner I created a PR which copies the properties and adds some additional checks.

I suspect that clone may not be used much - I would be surprised if anyone trips on the change in the semantics.