merge(~) feature for Composite State and Properties

[mxgrey]

Up until now, Composite::State and Composite::Properties has had a copy(~) function which will do a deep copy making the contents of one instance exactly the same as the contents of another instance.

This PR adds a merge function which will allow the receiving instance to hang onto any entries it has which are not present in the sending instance. So suppose we have:

Composite::Properties receiver, sender;
sender.create<Aspect1>();
receiver.create<Aspect2>();
receiver.merge(sender);

Then receiver will get the Aspect1 contents of sender, but it will also get to keep its own Aspect2 contents. On the other hand if we used receiver.copy(sender) instead, then receiver would lose its Aspect2 contents in order to be a perfect duplicate of sender.

I've also added a has<Aspect>() function for Composite::State and Composite::Properties.

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This change is

[jslee02]

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Reviewed 4 of 4 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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