Traceback inaccurate with gap open costs

[Daniel-Liu-c0deb0t]

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Need to keep track of traceback for the gap matrices (whether continuing or opening a gap). Currently, following gap transitions just end up on an entry in the dp matrix (not the gap matrices) even if its continuing a gap.

[RagnarGrootKoerkamp]

As an example, when aligning AC and AACC with cost model (sub=1, open=1, extend=1), triple_accel correctly returns cost 3, but the cigar is X=X= instead of =XX=.