`MergeMotifsAcrossRuns.ipynb` should account for indexes with respect to `--window`

[KnightAsterial]

Extension of #20, MergeMotifsAcrossRuns.ipynb creates a new seqlet set that combines the results from two different modisco runs. However, it does not take into account that tfmodisco-lite trims each input sequence around the center with respect to --window and labels start and end using the trimmed sequences. Thus, the recreated seqlets do not match the original seqlets.