Introduction of the net api for nodes

This implements a clean api for node function interaction with nodenets -- without the outward-facing functionality (dicts for serialization, load and save, etc), and slightly optimized for node function code elegance.
In the process, a list of native module issues have been fixed, so that native modules can now really make structural modifications to the node net without breaking things.

The net API is probably not final, and we should be liberal with introducing functionality there that would otherwise be copy and pasted into lots of native modules.

I am not happy with the way sensors access data sources -- too many indirections. The API object should directly expose the world adapater with sources and targets, the "world" object shouldn't be visible at all, and sensors should not have to pass in the ID of the nodenet they live in. Just not their job to be aware of this stuff.