Support sidre-to-conduit hierarchy copies

[gunney1]

Some tools and interfaces support only sidre hierarchies or conduit hierarchies. To use a combination of them, we need to switch seamlessly between the 2 hierarchy types. This means deep copying and shallow-copying between the two. Four methods needed (2 are currently supported):

1. Deep copy conduit to sidre: Group::importConduitNode
2. Deep copy sidre to conduit: Missing
3. Shallow copy conduit to sidre: Missing
4. Shallow copy sidre to conduit: Group::createNativeLayout

We should implement the missing methods.

Sidre and Conduit have ways to specify an allocator id for a hierarchy node. The deep copy should respect the value set for the destination node and propagate it down the destination hierarchy.

[nselliott]

For a shallow copy of conduit to sidre, the resulting sidre hierarchy would have to have all external views for its leaves, as the data would already be allocated and owned elsewhere, either by conduit or by someone else using conduit's external data support. This is not to discourage an implementation, as this should be useful functionality to provide more complete interoperability between the libraries, though anyone using it will need to exercise the extra care needed to avoid errors with the pointers to external data.