Also add some tweaks to the process-model script that I managed to lose.

Entities derived from models can now have volumes

This also updates the model-processing script to automatically
insert the model data into the appropriate source file.

I also significantly rewrote the process-potrace-model-forms.py
script, as well as the blender-model.py script. Most of the other
scripts are old; I removed some personal info from them, so they
don't work properly now.