Update implementation to reflect device specific graph changes

[EwanC]

Our implementation needs to be updated to match the changes introduced in spec PR #83
Most importantly, the API change to add a device to the graph.add() entry-point, but the error checks also need implemented.

[EwanC]

This issue is still valid in that the implementation still needs updated, however the revision of the spec to updated too should now reflect the outcome of PR #113