Switch to multi-node once launch can handle it

[stonier]

Presently, launch node name renaming is not possible via ros2 launch, so this enforces the decision to bundle everything into one node.

PROS:

• It's simpler

CONS:

• Constraints how the application writer might wish to execute (spin) the different parts of the system.
• Have to use the ugly **kwargs hack in setup

I'd like to revert to having behaviours take in node_name and namespace on init and construct their nodes themselves (supposing the overhead of nodes isn't large itself).

[stonier]

The one-node approach is working relatively well, no reason to fix something that ain't broke :)