Add GUI Nodes and Connections in a parent Network

[tcstewar]

Currently, all of the Nodes and Connections needed for the plots and other components of Nengo GUI are added to the top-level Network defined by the user (i.e. model). These are then removed after building the Simulator. In normal usage, this is fine, but in multi-threaded environments, it may be possible for another thread to access the model during the build process, and there'll be these odd extra Nodes and Connections.

To get around this (and to be a bit cleaner in general), we could change this algorithm from:

create_gui_components_in(model)
sim = nengo.Simulator(model)
remove_gui_components_from(model)

to this:

parent = nengo.Network()
parent.networks.append(model)
create_gui_components_in(parent)
sim = nengo.Simulator(parent)

Notice that this means that we don't have to remove the components afterward!

[jgosmann]

Tagging issue #870 which discusses the same problem, but proposes a different solution.

I wonder if this approach could make it easier to solve the problem that we have to rebuild the model when inserting graphs. Maybe it is possible to build model keep the built model around and use the already built signals when building the parent? Though, that makes some assumptions about how the model gets built that might not hold on all backends.