Documentation of the behaviour of the Tree and the Store 
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When you create a Tree it is associated with a Store 
(Maybe a dummy Store).

When you create a Factory it may be associated with a 
Tree (and thus implicitly with a Store).

Committing a Tree causes all the objects in that Tree 
to be stored.

Trees can contain "mount points" to other trees, which 
may have a different store. Mount points are not 
followed when storing trees. But the mount point itself 
is stored.

When the factory creates an Object which needs a Backing 
Store, such as NTuple and Cloud, it passes the the store
to the constructor. Other objects, such as Histograms do
NOT get the store passed into the constructor.

Symbolic links can only point within the same tree (they
cannot cross mount points).

We need a copy method to perform a deep-copy of part of 
the Tree.