Set sequencetools.iterate_sequence_nwise_*(..., n=2) default.

All three functions now default to n=2:

    * sequencetools.iterate_sequence_nwise_cyclic()
    * sequencetools.iterate_sequence_nwise_strict()
    * sequencetools.iterate_sequence_nwise_wrapped()

This means that these three functions now exhibit default behavior equal to the
following three functions:

    * sequencetools.iterate_sequence_pairwise_cyclic()
    * sequencetools.iterate_sequence_pairwise_strict()
    * sequencetools.iterate_sequence_pairwise_wrapped()

DEPRECATED: the three *_pairwise() functions are now deprecated. Use the
*_nwise() functions instead.