- added start end and direction params
- lots of minor cosmetic improvements
- added switching between assembly steps
 * next/previous step support , including on screen controls
 * basic support for loading/unloading resources for each step
 uses the build in caching of the usco-assetmanager
 * various minor related changes in all the custom elements

- added various test/demo stl files
- more experimentation with the "hierarchy" analysis for build step creation:
 * modified the "recurse" method to only return nodes with children,
 as you cannot assemble items in a flat hierarchy

- various tweaks and improvements (cosmetic, param names, nothing functional)