remove including of numpy headers
avoid copying ChartItems; include span length in agenda priority

- Copying ChartItem objects is avoided;
  The chart.probs dictionary is used to cons items to a unique instance
- ChartItems now have a field for the Viterbi probability
- When not using an outside estimate, the length of the span is now used
  as a simplistic estimate so that all spans of length n are explored
  before moving on to n + 1
fix print formatting
Update Makefile.

- include copy of valgrind-python.supp that adds suppresions for reads
  of size 8
- make faulthandler module optional
- fix bug with empty terminals and latex output.
- warn that specifying training set size as fraction is not possible
  with --rerun