Class dependant on copy modules

I duplicated the function int the class module.
node_clone_extra_properties_add() is just a helper function that makes it easy to add extra elements to a node, and groups the elements together on the node form. Since class properties should be in a separate group anyway duplicating the function makes sense.

Class_note and other class modules need this as well and shouldn't require node_copy to function. Since these modules all require the class module, this seemed like the best place to put it.

If the extra properties are all copy related, then this should use the copy module to add properties only if it is loaded.