Nested config contexts in the builder

[tbekolay]

Is your feature request related to a problem? Please describe.

Currently, build_network sets and unsets model.config so that build functions can query the config and alter build behavior accordingly.

However, Nengo core does use model.config (as far as I can recall) so there aren't examples of this happening in Nengo core. As a result, different backends have handled this differently. Nengo Loihi and (as far as I recall) Nengo SpiNNaker only ever read the top-level model's config to determine if the user wants an ensemble to be placed on/off chip. NengoDL does what most users would expect, which is that it reads the whole config stack to see if the settings for an object have been changed.

Describe the solution you'd like

Nengo core should adopt what NengoDL's done and introduce some convenience functions in the builder to make this easier to support in all backends.