Update PoC graph execution mechanism

[EwanC]

The current PoC code from #5 uses command_graph<graph_state::executable>::exec_and_wait() to execute a graph. This is from an outdated version of the graphs spec, and should be replaced by either queue::submit(command_graph<graph_state::executable> graph)
or handler::exec_graph(command_graph<graph_state::executable> graph)

See #4 (comment)

[EwanC]

Reassigning this to me so that I can update the execution mechanism to match handler::exec_graph from #26