Implement intersection for I/O transducers

[ihh]

The current intersection algorithm requires that both transducers be recognizers (i.e. empty output alphabets). But this is not necessary, because

• the current algorithm should work even if only one of the machines is a recognizer
• it is also possible to intersect machines at the alignment level (i.e. not just the I/O sequences but the alignment path must be aligned). This departs a little bit from the idea of machines as matrices indexed only by sequences, and would require some adjustment (e.g. the concept of a waiting machine might need to be modified in this case), but is useful e.g. in calculating likelihoods of particular alignments, constraining machines to follow a particular alignment path, and so on; and it may even generalize to alignment envelopes.