legalTagBigram(): disable enforcement of matching classes for
consecutive chunk members by default
sst2tags.py (adapted from mwe2tags.py)
update i_viterbi() to match the c_viterbi() API. i_viterbi() doesn't
seem to be any faster, though.
in decoding, iterate over 0-order percepts as outer loop, maintaining
total scores for all labels. fixes #7