Allow for time slippage between the two sources

[huffyhenry]

Add the difference between clocks of the last aligned pair to the the main dynamic programming array. For the next potential match, remove that difference when computing the clock component of the score. This way, we can incorporate situations where the F24 and Tracab clocks are shifted by a constant amount from some point onwards.

[huffyhenry]

Closing. In practice, the algorithm copes with slippage well without this correction. It could even be counter-productive to implement it, because slippage tends to disappear after the play slows down.