Factor out ``no-roots no-prefixes'' into its own word. Add load-root, load-from-root...

Factor out ``no-roots no-prefixes'' into its own word. Add load-root, load-from-root, vocabs-in-root and implement load in terms of load-from-root.
Add error checking for append-vocab-dir and (child-vocabs) in case someone tries to pass them the wrong parameters.
Rename: vocab-dir+ to append-vocab-dir
Document load-from-root and load-root