Implement executable graph update

[EwanC]

The void command_graph<graph_state::executable> update(const command_graph<graph_state::modifiable>& graph); feature included in the spec is not defined in our prototype, we should add this entry-point.

The depth of this implementation is up for discussion, options are:

• Stub entry-point which throws an exception saying not implemented
• Emulated functionality by recreating the underlying PI commands
• Extending PI to introduce argument update of existing commands, which this functionality is then built upon.

[EwanC]

Stubbed out entry-point added in #156

[EwanC]

Closing ticket as feature was implemented in intel#13220