Nodenets are code

Not so much a pull request as a request for comments and feedback

I've introduced the option of giving your own resource-location via a config.ini, which means the following:

* Nodenet and world data will be saved to this location
* you can define your own nodetypes via a simple json file called `nodetypes.json` in your resource-location
* you can define your own nodefunctions in `nodefunctions.py` in your resource-location
* no further configuration should be required.

still to be discussed:
* we still have hardcoded link-pairs: por/ret, sub/sur... - should we allow for definition of custom pairs as well?
* should the standard-nodetypes be fixed, or should we allow for overwriting standard-nodetypes in the custom definitions?
* probably some stuff that I forgot.

Please try this out, and hit me with feedback.