drop derivations that cannot be recovered (find out why)

- disambiguation: skip derivations that cannot be recovered (finding out
  why is a TODO).
- _grammar: lift requirement for sorted grammar
- runexp: make extra prob. models (.probs.npz file) optional
- Makefile: improve 'make clean'; don't call 'python setup.py clean'
  because that triggers cythonize() ...
- treeedraw: don't highlight the '...' ellipsis indicating a frontier
  non-terminal, to distinguish it from an actual '...' terminal.