54,56 compute stats improvements

Resolves #54 (closed): compute_stats.py automatically writes class exemplars (in pickle format) to output/<experiment name>/class-exemplars.pkl. Subsequent experiments can read this file to initialize the class exemplars. See new config setting class exemplar path in config/examples/stats-setup.ini

Resolves #56 (closed): Goal state labels, previously hard-coded in compute_stats.py, have been moved to config/goal-state-labels/. Each goal state has its own file.