Decide on graph default device

[EwanC]

Motivated by PR review comment #83 (comment)

With the change in a modifiable graph from being device-agnostic to device-specific, the explicit API now takes an extra device parameter to graph.add(). It may make users lives more ergonomic for the most common single-device use cases if the graph had a default device that nodes were created for, and there was an overload of graph.add() which didn't need the device and used the default device instead.

The open question with this is whether a user should be able to change the default device, or if it's immutable on graph construction.

If a user can't change the default device, then a property to the existing graph constructor would be fine for setting the
default device, since there is no set_property in SYCL. But it would be useful for a user to change the default device, then maybe a set_default_device()/get_default_device() pair of functions would be also be worth adding, as well as overloading the current constructor with command_graph(device) too.

[EwanC]

Closing, since we'll go with a different approach in revision 1